Add description

Author: yanecc

README.md

@@ -32,7 +32,14 @@ They are the same and will install Nightly Builds tagged with the current date.
 vfox use crystal@20240420
 ```
 
+Some environment variables are served as following:
+
+| Environment variables |     Default value     |   Description    |
+| --------------------- | --------------------- | ---------------- |
+| GITHUB_TOKEN          | ""                    | GitHub Token     |
+| GITHUB_URL            | "https://github.com/" | GitHub proxy URL |
+
 ## Note
 
 This plugin uses the GitHub API to retrieve available versions of Crystal. It is recommended to modify the `~/.version-fox/plugin/crystal/lib/util.lua` file to set your personal GitHub token for a higher rate limit before formal use.
-https://github.com/yanecc/vfox-crystal/blob/04c6c68b29ca33301a9005c5ddf9b3ea5bb3cb1c/lib/util.lua#L72-L73
+https://github.com/yanecc/vfox-crystal/blob/128cc5ed1c9c5a49d89b3a6f58ce686180d0af60/lib/util.lua#L72-L73