Document software development lifecycle

[hturner]

We could do with more documentation on the development lifecycle of R, see https://www.r-project.org/doc/R-SDLC.pdf, in particular explaining what the alpha, beta and RC (release candidate) version are. This is implicitly documented in https://developer.r-project.org/release-checklist.html. I think this could go in an extended version of the "Testing Pre-release R Versions", since it's important to understand how this testing fits in to the development lifecycle. This should also include the approximate timing (release around the end of April each year, dates published on https://developer.r-project.org/). It should also document that r-patched binaries will be replaced by the alpha/beta/RC versions when those are released, so you can follow the usual links for r-patched to install these versions (this isn't altogether clear from the R Core blog post, as you can only see the alpha/beta/RC versions when they are available!).