plugins/README.md: Initial plugin documentation

For now, only a few hints ...

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+AX-ZSH: Alex' Modular ZSH Configuration - Plugin Documentation
+==============================================================
+
+Plugin Exit Codes
+-----------------
+
+- 0: OK
+- 1: Error
+- 91: Ignore
+- 92: Optional
+
+
+Detecting "Plugin Check" Run
+----------------------------
+
+When `axzshctl check-plugins` is doing a _plugin check_ run, the enviromnet
+variable `AXZSH_PLUGIN_CHECK` is "non-zero"; this can be tested like this:
+
+```
+[[ -z "$AXZSH_PLUGIN_CHECK" ]] || return 92
+```