Newcomers_Guide.rst: Allow only one newcomer issue
Newcomers_Guide.rst: Allow only one newcomer issue

Tells the newcomer to merge only one difficulty/newcomer issue.

Closes https://github.com/coala/coala/issues/4771

coala/coala,jayvdb/coala,coala-analyzer/coala,karansingh1559/coala,SanketDG/coala,coala-analyzer/coala,jayvdb/coala,karansingh1559/coala,Nosferatul/coala,karansingh1559/coala,SanketDG/coala,coala-analyzer/coala,jayvdb/coala,Nosferatul/coala,coala/coala,Nosferatul/coala,coala/coala,SanketDG/coala