Document xcb-proto regeneration

* README.md: Document where to get the xcb-proto definitions and how to
  re-generate the `.el` files.

Author: Stebalien

README.md

@@ -6,3 +6,19 @@ It features an object-oriented API and permits a certain degree of concurrency.
 It should enable you to implement some low-level X11 applications.
 Please refer to [xelb.el](https://github.com/emacs-exwm/xelb/blob/master/xelb.el)
 for more details.
+
+## Regenerating XCB Bindings
+
+Most (although not all) bindings in this library are auto-generated from [xcb-proto][]. To regenerate them:
+
+1. Install your distro's `xcb-proto` package (`apt install xcb-proto`, `pacman -S xcb-proto`, etc.).
+2. Run `make`.
+
+Alternatively:
+
+1. Download the latest `xcb-proto` [release][xcb-proto-releases].
+2. Extract it.
+3. Run `make PROTO_PATH=/path/to/xcb-proto/src`
+
+[xcb-proto]: https://gitlab.freedesktop.org/xorg/proto/xcbproto
+[xcb-proto-releases]: (https://gitlab.freedesktop.org/xorg/proto/xcbproto/-/tags)