[Contributing] Add some welcoming words for new contributors

And the explicit permission to also do something as seemingly trivial
as naming functions or data on separate branches, which apparently
still needed to be said somewhere.

CONTRIBUTING.md

@@ -1,3 +1,22 @@
+## Welcome!
+
+If we've seen you doing any kind of reverse-engineering or modding work on the
+PC-98 Touhou games before, you might have already been [invited as a
+collaborator][2]. In that case, feel free to create separate branches for your
+work directly in this repository – this will immediately inform anyone who
+watches this repo or subscribed to a webhook. If you prefer, you can still
+use your own fork though.
+
+### What can I do on these separate branches?
+
+Anything – reverse-engineering and decompilation of original ZUN code (which
+then could be merged back into `master` after review) or your own custom mods,
+no matter how large or small.
+
+For starters, simply naming functions or global variables to reflect their
+actual intent will already be helpful. *Any* name is better than
+`sub_<something>`, and can always be fixed or improved later.
+
 # Contribution guidelines
 
 ## Rule #1
@@ -8,8 +27,6 @@ binary, as compared using [mzdiff].** The only allowed exceptions are:
 1) different encodings of identical x86 instructions within code segments
 2) padding with `00` bytes at the end of the file.
 
-Other branches are, of course, free to experiment with mods or refactorings.
-
 ## Taste issues
 
 * Use tabs for indentation.
@@ -70,3 +87,4 @@ Other branches are, of course, free to experiment with mods or refactorings.
 
 [mzdiff]: https://github.com/nmlgc/mzdiff
 [1]: Research/Borland%20C++%20decompilation.md#c
+[2]: https://github.com/nmlgc/ReC98/invitations