update release documentation (#490)

* update release documentation

Co-authored-by: Sean Barbeau <[email protected]>

RELEASE.md

@@ -1,14 +1,35 @@
 # Release instructions
 
-### 1. Do the release
+### 1. Do a pre relase
+1. Go to the release section of GitHub ![step 1](https://user-images.githubusercontent.com/35747326/99820876-567dd600-2b1f-11eb-87d2-eef132b3016a.png)
 
-Prepare a draft release on GitHub and **tag the master branch**
-here is a useful command to filter PRs
+1. Start a draft release ![step 2](https://user-images.githubusercontent.com/35747326/99822107-ce003500-2b20-11eb-9364-6dc8356e1276.png)
+1. Create a tag like `v1.3.2` and **use the current master branch**
+1. Leave the release description empty
+1. Check the `this is a pre release checkbox`
+1. Publish the prerelease
+![publish](https://user-images.githubusercontent.com/35747326/99821598-3ef31d00-2b20-11eb-9f5e-26f6583ad6c9.png)
+
+### 2. Do the release
+1. While CI is running to create the artifacts, you can work on the release description. ![edit](https://user-images.githubusercontent.com/35747326/99821184-ba080380-2b1f-11eb-8efe-57be80a0bd29.png)
+
+
+💡 Command to find PRs merged after a certain date:
 ```
 is:pr is:closed merged:>2020-07-28 base:master sort:updated-desc 
 ```
-Let the CI run and collect the artifacts
-Add them to the release
-Release
+2. When the CI is done, drag and drop the artifacts in the pre release assets section. You'll need to manually rename some of the assets for now (see below).
+
+‼️ **TODO**: The CI `.yml` files should be updated so that the output artifact names don't contain spaces and don't have to be manualy reworked when attached to a release (all space characters are replaced by . characters by GitHub).
+
+3. Once everything is ready, simply uncheck the `pre release` box and publish again ✅
+![publish](https://user-images.githubusercontent.com/35747326/99821105-99d84480-2b1f-11eb-9661-493966904a11.png)
+
+💡 For more details on versioning, see [Understanding Maven Version Numbers](https://docs.oracle.com/middleware/1212/core/MAVEN/maven_version.htm#MAVEN8855).
+
+### 3. Remove all `sha` Docker images added since last release
+1. Find the [list of Docker images for this project.](https://github.com/orgs/MobilityData/packages/container/gtfs-validator/versions)
+1. Delete all `sha`tagged Docker images added since last release.
+![grhc preview](https://user-images.githubusercontent.com/35747326/100006687-e1b5d080-2d98-11eb-846d-af12fbd7ca9f.png)
+**⚠️ Note: this manipulation can only be done by someone whose GitHub account has `Admin` access rights over the `gtfs-validator` package.** 
 
-For more details on versioning, see [Understanding Maven Version Numbers](https://docs.oracle.com/middleware/1212/core/MAVEN/maven_version.htm#MAVEN8855).