Updated README.md, other housekeeping changes.

Replaced LICENSE.txt with LICENSE.md because this is considered a best
practice at this time.
Bumped Version Number, Updated config.yaml, Briefly Tested with MT 4.38 and 5.2.2,

Bumped UnrecognizedTags to version 0.7.  Added links to Six Apart
website and to the movabletype/mt-plugin-unrecognized-tags repository
in Github to config.yaml.  Installed and briefly tested
UnrecognizedTags with MT 4.38 and 5.2.2.  Both displayed the message
"No unrecognized template tags could be found" as expected.
Merge pull request #3 from After6Services/master

Merge changes to documentation and config.yaml