Probably need to chown the executables again and improve docs for their updating

[Cervator]

I probably forgot to make sure groovyw and groovyw.bat were marked as executable when doing the last release. Additionally while I added a little note about how a usage of the wrapper may want to tweak the target script there's another bit of doc that could be added: how to maintain the two introduced executables.

groovyw was originally copied from gradlew and likewise groovyw.bat from gradlew.bat with a tiny adjustment to target Groovy scripts instead. While a lot of the stuff in the original files could be seen as fluff for what little the Groovy variant uses there may still be something causing a breaking change sooner or later. Much like explaining how to alter the scripts to a desired target exactly what was changed to make the scripts more Groovy would be another good note.