Add mage installation step to CONTRIBUTING.md

diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 3b4ee6329d61b34d5f9425126a91d5409eddf418..1ef37a69d2913173c0dd062f384ac2c957793666 100644 (file)
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -145,7 +145,13 @@ started:
 
 ### Build Hugo with Your Changes
 
-**Note:** Hugo uses [mage](https://github.com/magefile/mage) to build.
+**Note:** Hugo uses [mage](https://github.com/magefile/mage) to build. To install `mage` run
+
+```bash
+go get github.com/magefile/mage
+```
+
+`mage -l` lists all available commands with the corresponding description. To build Hugo run
 
 ```bash
 cd $HOME/go/src/github.com/gohugoio/hugo