docs(install): Add reference to hg in install docs

Also, hg is required.

docs/install.md

@@ -53,7 +53,7 @@ Here are links to the common builds:
 Building Helm from source is slightly more work, but is the best way to
 go if you want to test the latest (pre-release) Helm version.
 
-You must have a working Go environment with (https://github.com/Masterminds/glide)[glide] installed.
+You must have a working Go environment with (https://github.com/Masterminds/glide)[glide]and Mercurial installed.
 
 ```console
 $ cd $GOPATH