Add support of pre-release Python versions (#112)

* Add support of unstable Python versions

* Update README

* Get rid of stable boolean input

* Fix typo in the test.yml

* Update README

Co-authored-by: MaksimZhukov <v-mazhuk@microsoft.com>

README.md

@@ -16,6 +16,7 @@ This action sets up a Python environment for use in actions by:
 - Ability to download, install and set up Python packages from `actions/python-versions` that do not come preinstalled on runners
   - Allows for pinning to a specific patch version of Python without the worry of it ever being removed or changed
 - Automatic setup and download of Python packages if using a self-hosted runner
+- Support for pre-release versions of Python
 
 # Usage
 
@@ -92,6 +93,26 @@ jobs:
 
 ```
 
+Download and set up a accurate pre-release version of Python:
+```yaml
+steps:
+- uses: actions/checkout@v2
+- uses: actions/setup-python@v2
+  with:
+    python-version: '3.9.0-beta.4'
+- run: python my_script.py
+```
+
+Download and set up the latest available version of Python (includes both pre-release and stable versions):
+```yaml
+steps:
+- uses: actions/checkout@v2
+- uses: actions/setup-python@v2
+  with:
+    python-version: '3.9.0-alpha - 3.9.0' # SemVer's version range syntax
+- run: python my_script.py
+```
+
 # Getting started with Python + Actions
 
 Check out our detailed guide on using [Python with GitHub Actions](https://help.github.com/en/actions/automating-your-workflow-with-github-actions/using-python-with-github-actions).