Add READMEs to plugins copydir and copyfile (#6802)

* Add README to copydir plugin

* Add README to copyfile plugin

plugins/copydir/README.md

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+# copydir plugin
+
+Copies the path of your current folder to the system clipboard.
+
+To use, add `copydir` to your plugins array:
+```
+plugins=(... copydir)
+```
+
+Then use the command `copydir` to copy the $PWD.

plugins/copyfile/README.md

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+# copyfile plugin
+
+Puts the contents of a file in your system clipboard so you can paste it anywhere.
+
+To use, add `copyfile` to your plugins array:
+```
+plugins=(... copyfile)
+```
+
+Then you can run the command `copyfile <filename>` to copy the file named `filename`.