add topics coarray, coarray-fortran

[Beliavsky]

I suggest adding the topics coarray and coarray-fortran in the About section.

[nncarlson]

I was very confused about the "About" section, thinking you were suggesting I add some content to the README.md file. But it finally clicked what you are talking about -- I'd never really noticed that section of the web page before. Done. Thanks for the suggestion!