Newcomers_Guide: Correct the sign-up instructions
Newcomers_Guide: Correct the sign-up instructions

Update instructions for signing up in gitter, now that coalabot adds
new users rather than waiting for a maintainer. Sections 1 and 2
combined, all subsequent section numbers reduced by 1.

Closes https://github.com/coala/coala/issues/4251

shreyans800755/coala,jayvdb/coala,coala-analyzer/coala,SanketDG/coala,Asalle/coala,rimacone/testing2,SanketDG/coala,coala/coala,nemaniarjun/coala,aptrishu/coala,CruiseDevice/coala,karansingh1559/coala,coala/coala,Asalle/coala,CruiseDevice/coala,Nosferatul/coala,aptrishu/coala,kartikeys98/coala,rimacone/testing2,karansingh1559/coala,aptrishu/coala,Nosferatul/coala,jayvdb/coala,damngamerz/coala,CruiseDevice/coala,Nosferatul/coala,kartikeys98/coala,Asalle/coala,coala-analyzer/coala,shreyans800755/coala,damngamerz/coala,karansingh1559/coala,jayvdb/coala,nemaniarjun/coala,nemaniarjun/coala,damngamerz/coala,coala-analyzer/coala,SanketDG/coala,coala/coala,rimacone/testing2,shreyans800755/coala,kartikeys98/coala