Setup python + self hosted runners documentation

README.md

@@ -82,9 +82,26 @@ GitHub hosted runners have a tools cache that comes with Python + PyPy already i
 |**PyPy Tool Cache**|`RUNNER_TOOL_CACHE/PyPy/*`|
 
 GitHub virtual environments are setup in [actions/virtual-environments](https://github.com/actions/virtual-environments). During the setup, the available versions of Python and PyPy are automatically downloaded, setup and documented.
-- [Tools cache setup for Ubuntu](https://github.com/actions/virtual-environments/blob/master/images/linux/scripts/installers/1804/hosted-tool-cache.sh)
+- [Tools cache setup for Ubuntu](https://github.com/actions/virtual-environments/blob/master/images/linux/scripts/installers/hosted-tool-cache.sh)
 - [Tools cache setup for Windows](https://github.com/actions/virtual-environments/blob/master/images/win/scripts/Installers/Download-ToolCache.ps1)
 
+# Using `setup-python` with a self hosted runner
+
+If you would like to use `setup-python` on a self-hosted runner, you will need to download all versions of Python & PyPy that you would like and setup a similar tools cache locally for your runner.
+
+- Create an global environment variable called `AGENT_TOOLSDIRECTORY` that will point to the root directory of where you want the tools installed. The env variable is preferrably global as it must be set in the shell that will install the tools cache, along with the shell that the runner will be using.
+    - This env variable is used internally by the runner to set the `RUNNER_TOOL_CACHE` env variable
+    - Example for Administrator Powershell: `[System.Environment]::SetEnvironmentVariable("AGENT_TOOLSDIRECTORY", "C:\hostedtoolcache\windows", [System.EnvironmentVariableTarget]::Machine)` (restart the shell afterwards)
+-  Download the appropriate NPM packages from the [GitHub Actions NPM registry](https://github.com/orgs/actions/packages)
+    - Make sure to have `npm` installed, and then [configure npm for use with GitHub packages](https://help.github.com/en/packages/using-github-packages-with-your-projects-ecosystem/configuring-npm-for-use-with-github-packages#authenticating-to-github-package-registry)
+    - Create an empty npm project for easier installation (`npm init`) in the tools cache directory. You can delete `package.json`, `package.lock.json` and `node_modules` after all tools get installed
+    - Before downloading a specific package, create an empty folder for the version of Python/PyPY that is being installed. If downloading Python 3.6.8 for example, create `C:\hostedtoolcache\windows\Python\3.6.8`
+    - Once configured, download a specific package by calling `npm install`. Note (if downloading a PyPy package on Windows, you will need 7zip installed along with `7z.exe` added to your PATH)
+- Each NPM package has multiple versions that determine the version of Python or PyPy that should be installed. 
+    - `npm install @actions/toolcache-python-windows-x64@3.7.61579791175` for example installs Python 3.7.6 while `npm install @actions/toolcache-python-windows-x64@3.6.81579791177` installs Python 3.6.8
+    - You can browse and find all available versions of a package by searching the GitHub Actions NPM registry
+![image](https://user-images.githubusercontent.com/16109154/76194005-87aeb400-61e5-11ea-9b21-ef9111247f84.png)
+
 # Using Python without `setup-python`
 
 `setup-python` helps keep your dependencies explicit and ensures consistent behavior between different runners. If you use `python` in a shell on a GitHub hosted runner without `setup-python` it will default to whatever is in PATH. The default version of Python in PATH vary between runners and can change unexpectedly so we recommend you always use `setup-python`.