Added notes about CodeGen

Author: prioux

README.codegen.txt

@@ -0,0 +1,21 @@
+
+When generating this library using the online
+Swagger CodeGen, edit the CodeGen options for
+the PHP client:
+
+  composerProjectName: aces/cbrain-php-client
+  gitRepoBaseURL: https://github.com
+  gitRepoId: aces/cbrain-php-client
+
+This will allow you to generate exactly
+the library in the first commit of this repo.
+
+However, there are still a few modifications to
+make to the README.md and the composer.json.
+These are shown in the subsequent commits.
+
+I don't really undertand PHP and Composer,
+so I created this based on what I saw in
+the other old repo "xlecours/cbrain-php-client".
+
+Pierre Rioux, May 2022