Newcomers_Guide.rst: Add reference to GitLab repo
Newcomers_Guide.rst: Add reference to GitLab repo

Also mention about GitLab contributions being
counted for developer upgrade.

Closes #3901

coala-analyzer/coala,SanketDG/coala,coala/coala,coala/coala,jayvdb/coala,SanketDG/coala,jayvdb/coala,jayvdb/coala,coala-analyzer/coala,coala-analyzer/coala,coala/coala,SanketDG/coala