add initial contributing document

[cpanato]

What type of PR is this?
/kind documentation

Any specific area of the project related to this PR?

Uncomment one (or more) /area <> lines:

/area build

/area config

/area outputs

/area tests

/area helm

What this PR does / why we need it:
Add initial CONTRIBUTING.md document

Which issue(s) this PR fixes:

Fixes #96

Special notes for your reviewer:

[Issif]

Ok for starting with this file, we need to discuss with whole community about how we deal with contributor files

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Issif

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[poiana]

LGTM label has been added.

Git tree hash: 14d3201e98467dc638ef5960c6ba21485cf40e33

[cpanato]

@Issif how about have a topic in the falco community meeting?

[Issif]

@cpanato I agree, that was my first idea and we already talked about with @leogr