docs: add API and naming

[scop]

• Public interface and backward compatibility #537
• Customizability and naming conventions for each completion #539

[akinomyoga]

I think the descriptions of the existing rules are fine. In addition, I have questions about the naming rules for the following cases (I have created separate threads in #539):

• More flexible positions of double underscores in the names? -> #539 (thread)
• Naming of local variables in the function that exports variables of dynamic names? -> #539 (thread)

[akinomyoga]

#539 (reply in thread)

I think we have a conclusion here and in #791. Could you document the convention in #731? TIA :)

I'm not sure if I correctly get your intent, but I added a commit 4f6e424 that describes the rule of _mut_.

I also suggest using the word "private" instead of "internal" for the symmetry with "public" in the counterpart (see commit 5435b7c), though I am not sure if you like it. If not, we can drop this commit any time.

[scop]

Both 4f6e424 and 5435b7c look good to me, thanks!

[akinomyoga]

Thanks for checking!

[scop]

I wonder if we have some unfinished business here still, or should we proceed to merge?

[akinomyoga]

I think we can merge it. Maybe there would be something to adjust after the refactoring, but I think that can be always processed in another PR.

By the way, I think it is good to include links to these documents in CONTRIBUTING.md

[scop]

Link to CONTRIBUTING.md added, cheers.

[akinomyoga]

Thanks!