Add a list of contributors in README.md and remove AUTHORS file

This commit also adds AUTHORS and ChangeLog to .gitignore.
ChangeLog is only used by autoconf/automake and it has a quite
strict format. Since msgpack-c's change log is now written in
Markdown, it's difficult to follow the ChangeLog format.
So, unlike README.md, bootstrap doesn't copy CHANGELOG.md
to ChangeLog.

Author: nobu-k

.gitignore

@@ -1,5 +1,7 @@
 # Files generated by the bootstrap script.
 /INSTALL
+/AUTHORS
+/ChangeLog
 /NEWS
 /README
 /ac/

AUTHORS

@@ -210,6 +210,8 @@ Contributing
 To report an issue or send a pull request, use the
 [issue tracker](https://github.com/msgpack/msgpack-c/issues).
 
+Here's the list of [great contributors](https://github.com/msgpack/msgpack-c/graphs/contributors).
+
 License
 -------