add: Rule explanation in README

README.md

@@ -11,6 +11,39 @@ alias f="pay_respect"
 ```
 You can now **press `F` to Pay Respect**!
 
+## Rule Files
+
+Rule files are parsed at compilation. What actually gets compiled is a HashMap that contains patterns and suggestions for a specific command.
+
+Syntax of a rule file (placed under [rules](./rules)):
+```toml
+# this field should be the name of the command
+command = "world"
+
+# you can add as much section of like this as you like
+[[match_output]]
+# the suggestion of this section will be used for the following patterns of the command output
+pattern = [
+	"pattern 1",
+	"pattern 2",
+]
+# this will keep the change the second argument to `fix`
+suggest = "{{command[0]}} fix {{command[2:]}}"
+
+[[match_output]]
+pattern = [
+	"pattern 1",
+]
+# this will add a `sudo` before the command, without touching the rest
+suggest = "sudo {{command}}"
+```
+
+The placeholder is evaluated as following:
+
+- `{{command}}`: All the command without any modification
+- `{{command[1]}}`: The first argument of the command (the command itself has index of 0)
+- `{{command[2:5]}}`: The second to fifth arguments. If any of the side is not specified, them it defaults to the start (if it is left) or the end (if it is right).
+
 ## Current Progress
 
 Currently, only correction to `sudo` permission is implemented.