Rename README.rst to README about erfa

[eteq]

I assumed github wouldn't render the readme correctly in the front page if it doesn't have the appropriate extension. And I think it's more important to have a very readable github page than worry about confusing people for a moment (presumably they'll figure it out quick enough).

But it maybe be that github is smart enough to figure this out on its own. Have you seen any examples where this worked, @olebole? If so, I have no objection to changing the name.

[timj]

github won't render a README as RST. It will display it in plain text form.

Best you can do is use a soft link. Else leave it as is.

[olebole]

OK. No major problem with that.