Supported Platforms section is now really only about platforms and not
libraries. Libraries were moved to the Building BIND section.
We now have section for required libraries, and second with optional
features. Wordy explanations were taken verbatim from the original
README.md.
Converted using pandoc 2.14.2-9 on Arch Linux:
$ pandoc --shift-heading-level-by=-1 -f markdown -t rst README.md > doc/arm/build.rst
Plus hand-edit to remove sections other than Building BIND 9, remove
misindentation in section headers, and add a standard copyright header.