Update Windows install instructions for Cygwin

[Brian Oxley (binkley)]

Would you make a trivial update to your front page docs for installation under Cygwin? Out of the box $M2_HOME will be a Windows-path, not a Unix one, which causes tar some grief.

tar zxvf /home/Brian/profile/Downloads/maven-color-logback-1.3-bundle.tar.gz -C $(cygpath $M2_HOME)

Use cygpath to turn (for example) c:\lib\apache-maven-3.3.9 into /cygpath/c/lib/apache-maven-3.3.9.

[Jean-Christophe Gay]

I have moved instructions into the wiki and add a note about cygwin.

Feel free to to edit them if they're not clear enough!

Thanks.