[docs] Don't assume that Homebrew is always on macOS

Linuxbrew is available on linux and WSL

[ci skip]

README.md

@@ -32,7 +32,7 @@ The `hub` executable has no dependencies, but since it was designed to wrap
 
 #### Homebrew
 
-`hub` can be installed through [Homebrew](https://docs.brew.sh/Installation) on macOS:
+`hub` can be installed through [Homebrew/Linuxbrew](https://docs.brew.sh/Installation):
 
 ``` sh
 $ brew install hub

etc/README.md

@@ -1,6 +1,6 @@
 # Installation instructions
 
-## Homebrew on macOS
+## Homebrew
 
 If you're using Homebrew, just run `brew install hub` and you should be all set
 with auto-completion. The extra steps to install hub completion scripts outlined