*(last updated December 1, 2014)
Metasploit is actively supported by a community of hundreds of contributors and thousands of users world-wide. As a result, the accompanying documentation moves quite quickly.
The best source of documentation on Metasploit development is https://github.com/rapid7/metasploit-framework/wiki. There are many treasures there, such as:
…and many, many more.
If you are looking for API documentation, you may run
rake yard to
generate a navigatable view of the comment documentation used throughout
Metasploit, or visit https://rapid7.github.io/metasploit-framework/api
for a recently generated online version.