Add PR template with release notes

[adriangonz]

Fixes #1471

Changelog

• Add PR template with release-notes block
• Update docs with guidelines on release notes

[seldondev]

Wed Jul 8 09:36:57 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-2086/1.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2086 --build=1

[seldondev]

Wed Jul 8 09:37:09 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-2086/2.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2086 --build=2

[adriangonz]

/cc @cliveseldon @gsunner @RafalSkolasinski

[axsaucedo]

Nice one!
/approve

[seldondev]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: axsaucedo

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[axsaucedo]

/test lint

[seldondev]

Thu Jul 9 08:34:38 UTC 2020
The logs for [lint] [3] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2086/3.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2086 --build=3

[seldondev]

Thu Jul 9 08:52:19 UTC 2020
The logs for [pr-build] [4] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2086/4.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2086 --build=4

[seldondev]

Thu Jul 9 08:53:16 UTC 2020
The logs for [lint] [5] will show after the pipeline context has finished.
https://github.com/SeldonIO/seldon-core/blob/gh-pages/jenkins-x/logs/SeldonIO/seldon-core/PR-2086/5.log

impatient try
jx get build logs SeldonIO/seldon-core/PR-2086 --build=5