clipboard: additional explanation

Author: buhman

README.md

@@ -80,12 +80,14 @@ When you are done, press `q` once to stop recording.
 
 ### clipboard
 
-When set, by default, PB_CLIPBOARD will put the paste url in the X11 primary buffer.
+When set, by default, `PB_CLIPBOARD` will put the paste url in the X11 `primary` buffer. You can modify this behavior by setting `PB_CLIPBOARD_TOOL`, possibly with additional arguments.
 
 ```sh
 PB_CLIPBOARD=1 pb …
 ```
 
+For usability, you'll likely want to export this variable in your shell rc file if this is a feature you want to use regularly.
+
 ## advanced usage
 
 If you have more complex scripting needs (like capturing the uuid for