Add client checklist + doc (#1297)

* Add client checklist + doc

* Add versionning doc

* Update release.md

* Update release.md

* Update .github/ISSUE_TEMPLATE/release-client.md

Co-authored-by: Denis Pisarev <denis.pisarev@parity.io>

* Fix grumbles

Co-authored-by: Denis Pisarev <denis.pisarev@parity.io>

.github/ISSUE_TEMPLATE/release-client.md

@@ -0,0 +1,20 @@
+---
+name: Release Checklist for Client - issue template
+about: Release Checklist for Client
+title: Release Checklist - Client {{ env.VERSION }}
+---
+
+# Release Checklist - Client
+
+### Client Release
+
+- [ ] build a new `polkadot-parachain` binary and publish it to S3
+- [ ] new `polkadot-parachain` version has [run on the network](../../docs/release.md#burnin)
+    without issue for at least 12h
+- [ ] a draft release has been created in the [Github Releases page](https://github.com/paritytech/cumulus/releases) with the relevant release-notes
+- [ ] the [build artifacts](../../docs/release.md#build-artifacts) have been added to the
+    draft-release.
+
+---
+
+Read more about the [release documentation](../../docs/release.md).