Add initial RELEASE.md doc to describe release steps

RELEASE.md

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+# Release Procedure
+
+Here you can find some information about how to trigger a new release over PyPI and subsequent `conda-forge` update.
+
+## PyPI
+
+To release on PyPI you will need to create a new tag. To do so you can:
+
+* Create a [new GitHub release](https://github.com/napari/napari-plugin-manager/releases/new)
+* Use over the new release GitHub page the `Choose a tag` dropdown to create a new tag (it should be something like `vX.Y.Z` incrementing the major, minor or patch number as required).
+* Once the tag is defined you should be able to click `Generate release notes`.
+* Put as release title the tag that was created (`vX.Y.Z`).
+* Publish the release, check that the deploy step was run successfully and that the new version is available at [PyPI](https://pypi.org/project/napari-plugin-manager/#history)
+
+## conda-forge
+
+To update the `conda-forge` package you will need to update the [`napari-plugin-manager` feedstock](https://github.com/conda-forge/napari-plugin-manager-feedstock). **If a new version is already available from PyPI**, you can either wait for the automated PR or trigger one manually:
+
+* Create an issue over the feedstock with the title: [`@conda-forge-admin, please update version`](https://conda-forge.org/docs/maintainer/infrastructure/#conda-forge-admin-please-update-version)
+* Tweak the generated PR if necessary (dependencies changes for example).
+* Merge the generated PR.