New ruleset gitignore

[paulo-ferraz-oliveira]

Name

gitignore: similar in concept to elvis_config or rebar_config.

Brief Description

Elvis should detect non-conformant content for an opinionated list of elements that should be present in the .gitignore file (and that it exists, too).

For the moment (hoping a discussion ensues and we come to a "fixed" initial list)

.rebar3
_build
_checkouts
doc/
erl_crash.dump
rebar3.crashdump
test/logs/

Reasoning

We tend to forget one or the other, and this makes for easier analysis/consistency.