bug 1238587 - Add requirements for readthedocs.org
bug 1238587 - Add requirements for readthedocs.org

RTD breaks when installing packages against C libraries, so it doesn't
use any of the regular requirements files.

requirements-rtd.txt can be removed when the RTD configuration has been
updated.

mdn/browsercompat,jwhitlock/web-platform-compat,jwhitlock/web-platform-compat,mdn/browsercompat,mdn/browsercompat,jwhitlock/web-platform-compat