Add asdf installation instructions to docs (#383)

* Add asdf installation instructions to docs * Update site/content/index.md Co-authored-by: Nate Finch <natefinch@github.com> * Clean up asdf installation instructions Co-authored-by: Nate Finch <nate.finch@gmail.com> Co-authored-by: Nate Finch <natefinch@github.com>

Author: mathew-fleisch

site/content/index.md

@@ -57,6 +57,16 @@ See [mage homebrew formula](https://formulae.brew.sh/formula/mage).
 
 See [scoop](https://scoop.sh/).
 
+### Using asdf
+
+The [asdf version manager](https://asdf-vm.com/) is a tool for installing release binaries from Github. With asdf installed, the [asdf plugin for mage](https://github.com/mathew-fleisch/asdf-mage) can be used to install any released version of mage.
+
+```shell
+asdf plugin add mage
+asdf install mage latest
+asdf global mage latest
+```
+
 ## Example Magefile
 
 ```go