Create ADR for integrating cache functionality to setup-python action

docs/adrs/0000-caching-dependencies.md

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+## 0. Caching dependencies
+
+Date: 2021-10-01
+
+Status: Proposed
+
+## Context
+
+`actions/setup-python` is one the most popular python's action in GitHub Actions. A lot of customers use it in conjunction with `actions/cache` to speed up dependencies installation.
+See more examples on proper usage in [actions/cache documentation](https://github.com/actions/cache/blob/main/examples.md#python---pip).
+
+## Goals & Anti-Goals
+
+Integration of caching functionality into `actions/setup-python` action will bring the following benefits for action users:
+ - Decrease the entry threshold for using the cache for Python dependencies and simplify initial configuration
+ - Simplify YAML pipelines because no need additional steps to enable caching
+ - More users will use cache for Python so more customers will have fast builds!
+
+We will add support for Pip and Pipenv dependencies caching.
+
+We don't pursue the goal to provide wide customization of caching in scope of `actions/setup-python` action. The purpose of this integration is covering ~90% of basic use-cases. If user needs flexible customization, we should advice them to use `actions/cache` directly.
+
+## Decision
+
+ - Add cache input parameter to actions/setup-python. For now, input will accept the following values:
+    - pip - enable caching for pip dependencies
+    - pipenv - enable caching for pipenv dependencies
+    - '' - disable caching (default value)
+ - Cache feature will be disabled by default to make sure that we don't break existing customers.
+ - Action will try to search dependencies files (requirements.txt for pip and Pipfile.lock for pipenv) in the repository root (or relative to the repository root, if patterns are used) and throw error if no one is found.
+ - The hash of found file will be used as part of cache key (the same approach like actions/cache recommends)
+ - The following cache key will be used for pip: `setup-python-${{ runner.os }}-pip-${{ hashFiles('<package-file-path>') }}`
+ - The following cache key will be used for pipenv: `setup-python-${{ runner.os }}-python-${{ python-version }}-pipenv-${{ hashFiles('<package-file-path>') }}`. We add python version to the cache key, because virtualenv create folder with project name containing a copy of the python binary itself as a symlink to paths like `/opt/hostedtoolcache/Python/3.7.11`, so cache can be fetched with wrong python version. See details in the related [pull request](https://github.com/actions/cache/pull/607) in the actions/cache.
+ - Action will save the packages global cache:
+    - Pip (retrieved via pip cache dir). The command is available With pip 20.1 or later. We always update pip during installation, that is why this command should be available.
+    - Pipenv (default cache paths):
+        - ~/.local/share/virtualenvs (macOS)
+        - ~/.virtualenvs (Windows)
+        - ~/.local/share/virtualenvs (Ubuntu)
+ - Add `cache-dependency-path` input parameter to actions/setup-python. Input will accept array or regex of dependency files. The field will accept a path (relative to repository root) to dependencies file/files. If provided path contains wildcards, the action will search all maching files and calculate common hash like `${{ hashFiles('**/requirements-dev.txt') }}` YAML construction does
+
+## Example of real use-cases
+
+ - Pip package manager
+
+```
+steps:
+- uses: actions/checkout@v2
+- uses: actions/setup-python@v2
+  with:
+    python-version: 3.9
+    cache: pip
+```
+
+ - Pipenv package manager
+
+```
+steps:
+- uses: actions/checkout@v2
+- uses: actions/setup-python@v2
+  with:
+    python-version: 3.9
+    cache: pipenv
+```
+- With `cache-dependency-path`
+
+```
+steps:
+- uses: actions/checkout@v2
+- uses: actions/setup-python@v2
+  with:
+    python-version: 3.9
+    cache: pip
+    cache-dependency-path: **/requirements-dev.txt
+```
+
+## Release process
+
+As soon as functionality is implemented, we will release minor update of action. No need to bump major version since there are no breaking changes for existing users. After that, we will update [starter-workflows](https://github.com/actions/starter-workflows/blob/main/ci/python-app.yml) and [GitHub Action documentation](https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-python#caching-dependencies).