Updates to README.md

[silviubogan]

Through my attempt to build the docs on a Windows 10 computer resulted in these shortcuts that I think if put in README.md may help others too:

• Specify the first (or one of the first) Python 2.7 versions that includes pip.
• Specify install instructions of Sphinx using pip and the warning shown when installing that Sphinx supports Python 2 only till January of 2020.
• Specify .\ as a possibly necessary beginning of the docs-building command.