put insructions inside .envrc

.envrc

@@ -1,3 +1,12 @@
 #!/usr/bin/env bash
 
+#### Description
+# Direnv is a tool for user to separate their shell config into different folder specific profile. When `cd`ing into the `oil` folder, if direnv is allowed to run the script, it will automatically run the `build/dev-shell.sh` script. and dump environment variables into current shell.
+
+#### Usage
+# - Install direnv and its shell hook on https://direnv.net/#getting-started
+# - cd into git repo root
+# - Run `direnv allow`
+# - Now whenever you cd into oil repo or one of its subdirs, direnv will load environment variables directly for you.
+
 . build/dev-shell.sh