Add a deprecation section to the Contributing notes (#2248)

A brief explanation of the Deprecation Checklist is added to the
Contributing notes with a link to it

docs/contributor/CONTRIBUTING.md

@@ -152,3 +152,9 @@ We use [zepter](https://github.com/ggwpez/zepter) to enforce features are propag
 If you're member of **paritytech** org - you can use command-bot to run various of common commands in CI:
 
 Start with comment in PR: `bot help` to see the list of available commands.
+
+
+## Deprecating code
+
+When deprecating and removing code you need to be mindful of how this could impact downstream developers. In order to
+mitigate this impact, it is recommended to adhere to the steps outlined in the [Deprecation Checklist](./DEPRECATION_CHECKLIST.md).

docs/contributor/DEPRECATION_CHECKLIST.md

@@ -1,9 +1,7 @@
 # Deprecation Checklist
 
-This deprecation checklist makes sense while we don’t use [SemVer](https://semver.org/).
-After that, this document will most likely change.
-As deprecation and removal of existing code can happen on any release, we need to be mindful that external builders
-could be impacted by the changes we make.
+Polkadot SDK is under constant development and improvement, thus deprecation and removal of existing code happen often.
+When creating a breaking change we need to be mindful that external builders could be impacted by this.
 The deprecation checklist tries to mitigate this impact, while still keeping the developer experience, the DevEx, as
 smooth as possible.