Document action's fetch depth requirement (#11)

The changelog is generated from the commit history. But the ubiquitous `actions/checkout` action does a shallow fetch by default. This works perfectly well for almost every usage, so many users may not even be aware of the shallowness of the resulting repository or the ability to control the behavior via that action's `fetch-depth` input.

If you use the default `actions/checkout` configuration in your workflow, the resulting will contain, at most, a single entry. This can be extremely confusing. So the users will likely find some guidance in the required checkout configuration to be valuable.

Author: per1234

README.md

@@ -8,8 +8,30 @@ If no property is set the changelog will be created from the current commit to t
 
 ## Usage
 
+### Environment
+
+#### Working directory
+
 This action is meant to be launched inside a Git repository, thus the current `working-directory` must be set accordingly or it will fail.
 
+#### Checkout depth
+
+Because the changelog is generated from commit history, the action will not work as expected with a shallow fetch of the repository.
+So the [`actions/checkout`](https://github.com/actions/checkout) action's default fetch depth of 1 is not suitable.
+Set that action's [`fetch-depth` input](https://github.com/actions/checkout#fetch-all-history-for-all-tags-and-branches) to `0` to fetch the full commit history:
+
+```yaml
+      - name: Checkout repository
+        uses: actions/checkout@v2
+        with:
+          fetch-depth: 0
+
+      - name: Create Changelog
+        uses: arduino/create-changelog@v1
+```
+
+### Inputs
+
 The action accepts some properties:
 
 - `tag-regex` to pick which tags are taken into consideration to create the changelog, the example below would ignore all tags except those matching it, `0.0.1` would be accepted but `v0.0.1` or `0.0.1-rc` would be ignored.