Document the process for updating the registry

This avoids each maintainer to transmit the knowledge orally to the next
maintainer.

registry/README.md

@@ -0,0 +1,13 @@
+## Updating the registry XML
+
+In order to update the registry XML files and retain the history you cannot
+simply download the files the [Khronos website](https://khronos.org/registry/OpenGL/index_gl.php)
+and copy them into this directory. You should follow these steps, instead:
+
+ 1. check out the `khronos-registry` branch
+ 2. download the XML files from the Khronos repository
+ 3. copy them under the `registry` directory
+ 4. check the result for consistency and commit it
+ 5. check out the `master` branch and merge the `khronos-registry` branch
+    into it with the appropriate commit message
+