[CI] automate release process

[v1v]

What

Update docs with the contributing guideline. Add release guideline. Enable release automation when a tag release is created with the format d+.\d+.\d+ Release automation:

• will create a release in github matching the tag.
• will create a changelog using the gren app, filtering only the Pull Requests with certain labels:
  • "Breaking changes" category for all the PRs with the breaking label.
  • "Bug fixes" category for all the PRs with the bug or fix labels.
  • "Features" category for all the PRs with the enhancement, feature or feat labels.

Actions

• [x] Validate scripts locally

How to run locally

$ GITHUB_TOKEN=${GREN_GITHUB_TOKEN} TAG_NAME=1.0.2 make -f .ci/Makefile release

Output

Getting gren config from local file /app/.ci/.grenrc.js

🤖  - Generate release notes:
===================================
✔ Releases found: 3
✔ Tags found: 1.0.2
✔ Pull Requests found: 13
✔ 1.0.2 has been successfully created!
See the results here: https://github.com/elastic/ecs-logging-php/releases/tag/1.0.2

Tests

[apmmachine]

:green_heart: Build Succeeded

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#### Build stats * Build Cause: `Branch indexing` * Start Time: 2021-02-09T19:43:13.958+0000 * Duration: 15 min 37 sec * Commit: a79d7b952ce20fc22f51e6c64356ed1a5106962f #### Test stats :test_tube: | Test | Results | | ------------ | :-----------------------------: | | Failed | 0 | | Passed | 80 | | Skipped | 4 | | Total | 84 | #### Trends :test_tube:  

[v1v]

Somehow the current implementation with the gren library seems to be a bit buggy, so I'll try something else then