Specification definition discussion

[ifraixedes]

Please read the README and anything that you would like to discuss about the specification post a commend here.

[ifraixedes]

@GitEvents/owners check branch v0.x; I've implemented the first version of mailer plugin, however it needs test, which I will implement when as soon I get more time to spend in this.

I've updated the README with the reason of the changes which I've found when I started the implementation and I could do more thoughts and why the first spec didn't fulfil what I think that gitevents microservice needs.

[ifraixedes]

Test created and PR sent on #8