Add guidelines for experimental CodeQL queries and libraries

CONTRIBUTING.md

@@ -46,6 +46,8 @@ Follow the steps below to help other users understand what your query does, and
    Query help files explain the purpose of your query to other users. Write your query help in a `.qhelp` file and save it in the same directory as your new query. 
    For more information on writing query help, see the [Query help style guide](https://github.com/Semmle/ql/blob/master/docs/query-help-style-guide.md).
 
+In addition to contributions to our standard queries and libraries, we also welcome contributions of a more experimental nature, which do not need to fulfill all the requirements listed above. See the guidelines for [experimental queries and libraries](docs/experimental.md) for details.
+
 ## Using your personal data
 
 If you contribute to this project, we will record your name and email

cpp/ql/src/experimental/README.md

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+This directory contains [experimental](../../../../docs/experimental.md) CodeQL queries and libraries.

cpp/ql/test/experimental/README.md

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+This directory contains tests for [experimental](../../../../docs/experimental.md) CodeQL queries and libraries.

csharp/ql/src/experimental/README.md

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+This directory contains [experimental](../../../../docs/experimental.md) CodeQL queries and libraries.

csharp/ql/test/experimental/README.md

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+This directory contains tests for [experimental](../../../../docs/experimental.md) CodeQL queries and libraries.

docs/experimental.md

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+# Experimental CodeQL queries and libraries
+
+In addition to our standard CodeQL queries and libraries, this repository may also contain queries and libraries of a more experimental nature. Experimental queries and libraries can be improved incrementally and may eventually reach a sufficient maturity to be included in our standard libraries and queries.
+
+## Requirements
+
+1. **Directory structure**
+
+    - Experimental queries and libraries are stored in the `experimental` subdirectory within each language-specific directory in the [CodeQL repository](https://github.com/Semmle/ql). For example, experimental Java queries and libraries are stored in `ql/java/ql/src/experimental` and any corresponding tests in `ql/java/ql/test/experimental`.
+    - The structure of an `experimental` subdirectory mirrors the structure of standard queries and libraries (or tests) in the parent directory.
+
+2. **Query metadata**
+
+    - The query `@id` must not clash with any other queries in the repository.
+    - The query must have a `@name` and `@description` to explain its purpose.
+    - The query must have a `@kind` and `@problem.severity` as required by CodeQL tools.
+
+    For details, see the [guide on query metadata](https://github.com/Semmle/ql/blob/master/docs/query-metadata-style-guide.md).
+
+3. **Formatting**
+
+    - The queries and libraries must be [autoformatted](https://help.semmle.com/codeql/codeql-for-vscode/reference/editor.html#autoformatting).
+
+4. **Compilation**
+
+    - Compilation of the query and any associated libraries and tests must be resilient to future development of the standard libraries. This means that the functionality cannot depend on the output of `getAQlClass` and it cannot use internal APIs.
+    - The query and any associated libraries and tests must not cause any compiler warnings to be emitted (such as use of deprecated functionality or missing `override` annotations).
+
+5. **Results**
+
+    - The query must have at least one true positive result on some revision of a real project.
+
+6. **Contributor License Agreement**
+
+    - The contributor can satisfy the [CLA](CONTRIBUTING.md#contributor-license-agreement).
+
+## Non-requirements
+
+Other criteria typically required for our standard queries and libraries are not required for experimental queries and libraries. In particular, fully disciplined query [metadata](docs/query-metadata-style-guide.md), query [help](docs/query-help-style-guide.md), tests, a low false positive rate and performance tuning are not required (but nonetheless recommended).
+
+Experimental queries and libraries may not be actively maintained as the standard libraries evolve and may be removed in the future.
+

java/ql/src/experimental/README.md

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+This directory contains [experimental](../../../../docs/experimental.md) CodeQL queries and libraries.

java/ql/test/experimental/README.md

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+This directory contains tests for [experimental](../../../../docs/experimental.md) CodeQL queries and libraries.

javascript/ql/src/experimental/README.md

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+This directory contains [experimental](../../../../docs/experimental.md) CodeQL queries and libraries.

javascript/ql/test/experimental/README.md

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+This directory contains tests for [experimental](../../../../docs/experimental.md) CodeQL queries and libraries.

python/ql/src/experimental/README.md

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+This directory contains [experimental](../../../../docs/experimental.md) CodeQL queries and libraries.

python/ql/test/experimental/README.md

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+This directory contains tests for [experimental](../../../../docs/experimental.md) CodeQL queries and libraries.