Generate README-build.html from markdown

The build documentation is currently available in README-build.html. The problem with having the build instructions in html format is two-fold:

1) It is hard to read from a terminal

2) It is unnecessary difficult to modify

At least 2) means that it is an extra threshold to get over, in order to update or rewrite the build documentation.

Instead, we should have the documentation in markdown format, which is easy to read in a console even in source format, and is easy to modify. From this source format, a html version can easily be generated.