Getting_Involved: Shorten newcomer issues URL
Getting_Involved: Shorten newcomer issues URL

Change URL of newcomer issues in newcomer documentation
to new shorter URL "http://tinyurl.com/coala-new"
as it is easier to remember.

Fixes https://github.com/coala-analyzer/coala/issues/1980

dagdaggo/coala,dagdaggo/coala,dagdaggo/coala