Add notes on releasing the package

Author: Matt Mcveigh

RELEASE.md

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+# Releasing a new version of FSharpLint
+
+1. Update the [changelog](CHANGELOG.md) since last release and get the changes onto master.
+2. Tag the head of master with the version number in the format `vx.x.x` - for example: `v0.20.2`. 
+3. Push the tag to remote.
+
+After pushing the tag to remote, the publish pipeline will be kicked off which will:
+* Build the package
+* Create a github release
+* Pushes the package to NuGet
+* Publishes the docs to github pages
+
+## Dependencies
+
+* Github secret: `NUGET_KEY`.
+  * This is the API key from nuget, used to push the package to nuget.
+  * At the moment the key lives under `duckmatt`'s nuget account, but I believe the `fsprojects` account should also be able to create an applicable API key.
+  * "Expires in 7 months" from 2021/11/16