Allow redefining or augmenting information in the "INTERESTED IN DEVELOPMENT?" section

[jtsternberg]

Currently:
Browse the code, check out the SVN repository, or subscribe to the development log by email or RSS.

We should really be able to override this text and these links since:

• Development is almost always on Github
• Github's code-browsing features are more intuitive and familiar to most developers
• Github provides it's own methods for subscribing to development progress
• Github provides it's own contribution documentation recommendation through the CONTRIBUTING.md -- we should be able to link to that here.

Unfortunately, I think most people are trained to ignore that section, and instead look in the readme sections for contribution information, but it seems obvious we should make this section more realistic, intuitive, and configurable by plugin authors. And if not configurable, at least providing the ability to add more text below it by way of another readme section.

[SergeyBiryukov]

Related: #644, #3937.