Convert all README files to Markdown

A mindless task for a lazy weekend: convert all the README and
README.txt files to Markdown.  Paired with the slow conversion of all
of our man pages to Markdown, this gives a uniform language to the
Open MPI docs.

This commit moved a bunch of copyright headers out of the top-level
README.txt file, so I updated the relevant copyright header years in
the top-level LICENSE file to match what was removed from README.txt.

Additionally, this commit did (very) little to update the actual
content of the README files.  A very small number of updates were made
for topics that I found blatently obvious while Markdown-izing the
content, but in general, I did not update content during this commit.
For example, there's still quite a bit of text about ORTE that was not
meaningfully updated.

Signed-off-by: Jeff Squyres <jsquyres@cisco.com>
Co-authored-by: Josh Hursey <jhursey@us.ibm.com>

Author: jsquyres