Add release notes template

[adriangonz]

What this PR does / why we need it:
Fetch the release-notes block of every PR merged between 2 particular tags to generate easy-to-follow release notes.

Which issue(s) this PR fixes:

Fixes #1471

Special notes for your reviewer:
This is mostly a draft PR showing an example format which we could use to generate our release notes. The main goal is to discuss and get people's opinions on the template and how we could integrate them further with our release process.

Does this PR introduce a user-facing change?:

NONE

[seldondev]

Fri Jul 24 13:15:30 UTC 2020
The logs for [pr-build] [1] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2190/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2190 --build=1

[seldondev]

Fri Jul 24 13:16:09 UTC 2020
The logs for [lint] [2] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2190/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2190 --build=2

[adriangonz]

/cc @gsunner @axsaucedo @cliveseldon

[adriangonz]

If we are happy with the template for release notes, we can merge this PR and then enable the Prow / Lighthouse plugin to make sure all PRs have a release-notes block (or have explicitly disabled).

[ukclivecox]

/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: cliveseldon

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