Add markdown file extension

[elykwilliams]

Change file extension of README

Issue:
Currently, the README uses markdown formatting that is not recognized by github.

Proposed changes:
By adding the .md extension to README, github will detect and format the README appropriately.

[cburstedde]

Thanks. I'd be leaning towards maintaing a short README file with just the license and telling readers to read README.md for more details. So README.md would in this PR be a straight copy of the previous README.

Then we should make sure to distribute both files with both autoconf and CMake, and verify make distcheck and cpack.

[cburstedde]

While you're at it, may I ask to add a few lines to doc/release_notes.txt? We should rename it to doc/sc_release_notes.txt.

[cburstedde]

Thanks. I'd be leaning towards maintaing a short README file with just the license and telling readers to read README.md for more details. So README.md would in this PR be a straight copy of the previous README.

The copying is done, this is cool. Adding a stub README would be good to have due to tradition/convention.

Then we should make sure to distribute both files with both autoconf and CMake, and verify make distcheck and cpack.

For automake, adding README.md to the EXTRA_DIST variable would be required.

[cburstedde]

So, to do this consistently across both p4est and libsc, it would be nice to keep README for GNU conformance, maybe just as a stub referencing a more extensive README.md. Plus we may take a step back and move some of the README contents into the doxygen pages, shortening README and adding various links. This requires a documentation bump on the web first.

[cburstedde]

Ok I'm on board moving to README.md. If you'd like to check that the file is included and works from an autoconf 'make distcheck'? And if you'd like to fix whatever formatting errors we haven't noticed. I'll merge asap! Thanks again.