textmate: add README (#7267)

plugins/textmate/README.md

+# TextMate plugin
+
+The plugin adds a function for the [TextMate](https://macromates.com) editor.
+
+To use it, add `textmate` to the plugins array of your zshrc file:
+```
+plugins=(... textmate)
+```
+
+## Function
+
+The `tm` function provides the following options:
+
+- No arguments: Run `mate` in the current directory.
+- Argument that is a directory: Run `mate` in the given directory and cd to it.
+- Other arguments: Pass all arguments to `mate`. This allows for easy opening of multiple files.