docs: describe scopes in the contributing guidelines

The scope of a commit message should be the name of the NPM package affected by the change.
The following scopes are allowed:
- `toolkit`: If the change affects the `@scion/toolkit` NPM package.
- `ɵtoolkit`: If the change affects the `@scion/toolkit.internal` NPM package. We use the Theta (`ɵ`) symbol to have a shorter scope name.

.github/PULL_REQUEST_TEMPLATE.md

@@ -2,8 +2,8 @@
 Please check if your PR fulfills the following requirements:
 
 - [ ] The commit message follows our [guidelines](https://github.com/SchweizerischeBundesbahnen/scion-toolkit/blob/master/CONTRIBUTING.md)
-- [ ] Tests for the changes have been added (for bug fixes / features)
-- [ ] Docs have been added / updated (for bug fixes / features)
+- [ ] Tests for the changes have been added (for bug fixes and features)
+- [ ] Docs have been added / updated (for bug fixes and features)
 
 
 ## PR Type
@@ -15,7 +15,7 @@ What kind of change does this PR introduce?
 - [ ] Feature (new feature)
 - [ ] Documentation (changes to the documentation)
 - [ ] Refactoring (changes that neither fixes a bug nor adds a feature)
-- [ ] Performance (changes that improves performance)
+- [ ] Performance (changes that improve performance)
 - [ ] Test (adding missing tests, refactoring tests; no production code change)
 - [ ] Chore (other changes like formatting, updating the license, updating dependencies, removal of deprecations, etc)
 - [ ] CI (changes to our CI configuration files and scripts)

CONTRIBUTING.md

@@ -92,10 +92,10 @@ In additional to the linting rules, we have the following conventions:
 
 We believe in a compact and well written Git commit history. Every commit should be a logically separated changeset. We use the commit messages to generate the changelog.
 
-Each commit message consists of a **header**, a **body** and a **footer**.  The header has a special format that includes a **type** and a **subject**, as following:
+Each commit message consists of a **header**, a **summary** and a **footer**.  The header has a special format that includes a **type**, an optional **scope**, and a **subject**, as following:
 
 ```
-<type>: <subject>
+<type>(<scope>): <subject>
 
 [optional summary]
 
@@ -109,14 +109,23 @@ Each commit message consists of a **header**, a **body** and a **footer**.  The
 - `fix`: bug fix
 - `docs`: changes to the documentation
 - `refactor`: changes that neither fixes a bug nor adds a feature
-- `perf`: changes that improves performance
+- `perf`: changes that improve performance
 - `test`: adding missing tests, refactoring tests; no production code change
 - `chore`: other changes like formatting, updating the license, updating dependencies, removal of deprecations, etc
 - `ci`: changes to our CI configuration files and scripts
 - `revert`: revert of a previous commit
 - `release`: publish a new release
 </details>
 
+<details>
+  <summary><strong>Scope</strong></summary>
+
+The scope should be the name of the NPM package affected by the change.
+
+- `toolkit`: If the change affects the `@scion/toolkit` NPM package.
+- `ɵtoolkit`: If the change affects the `@scion/toolkit.internal` NPM package. We use the Theta (`ɵ`) symbol to have a shorter scope name.
+</details>
+
 
 <details>
   <summary><strong>Subject</strong></summary>