docs(README): improve action description

The description hasn't really been updated in a long time. So many of
the things written there were based on our assumptions when we started
the project.

We now know much better what the action does and what problem it solves.

Other actions have also improved their descriptions a lot. This new
description is inspired by some popular other actions, using our current
knowledge of the backport-action.

README.md

@@ -1,9 +1,17 @@
 # Backport action
 
-This is a GitHub action to backport merged pull requests (PR) to branches.
-For example, to patch an older version with changes that you're merging into your main branch, without the manual labor of cherry-picking the individual commits.
+A GitHub action to backport merged pull requests to selected branches.
 
-A backport consists of creating a new branch, cherry-picking the changes of the original PR and creating a new PR to merge them.
+This can be useful when you're supporting multiple versions of your product.
+After fixing a bug, you may want to apply that patch to the other versions.
+The manual labor of cherry-picking the individual commits can be automated using this action.
+
+The backport action will look for backport labels (e.g. `backport release-3.4`) on your merged pull request.
+For each of those labels:
+1. fetch and checkout a new branch from the target branch (e.g. `release-3.4`)
+2. cherry-pick the merged pull request's commits
+3. create a pull request to merge the new branch into the target branch
+4. comment on the original pull request about its success
 
 This backport action is able to deal with so called `octopus` merges (i.e. merges of multiple branches with a single commit).
 Therefore, this action is compatible with [Bors](https://bors.tech/) and similar tools.
@@ -17,25 +25,6 @@ Therefore, this action is compatible with [Bors](https://bors.tech/) and similar
 
 ## Usage
 
-Simply mark a PR with backport labels: `backport <branchname>`.
-
-For example, a PR with labels
-
-```
-backport stable/0.24
-backport release-0.23
-```
-
-will be backported to branches `stable/0.24` and `release-0.23` when merged.
-
-If something goes wrong, the bot will comment on your PR.
-It will also comment after successfully backporting.
-Links are created between the original and the new PRs.
-
-It's also possible to configure the bot to trigger a backport using a comment on a PR.
-
-## Installation
-
 Add the following workflow configuration to your repository's `.github/workflows` folder.
 
 ```yaml