tig: add more info in README (#8818)

For more convenient plugin exploring.

plugins/tig/README.md

 # `tig` plugin
 
-This plugin adds some aliases for people who work with `tig` in
+This plugin adds some aliases for people who work with [`tig`](https://jonas.github.io/tig/) (text-mode interface for Git) in
 a regular basis. To use it, add `tig` to your plugins array:
 
 ```zsh