doc: Propose conventional commits scopes.

CONVENTIONAL_COMMITS.md

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+# Conventional Commits
+
+This project uses [Conventional
+Commits](https://www.conventionalcommits.org/). Read the
+[Summary](https://www.conventionalcommits.org/en/v1.0.0/#summary) or
+the [Full
+Specification](https://www.conventionalcommits.org/en/v1.0.0/#specification)
+to learn more.
+
+## Scopes
+
+Conventional Commits defines _scope_ (as in `type(scope): message`). This
+section aims at listing all the scopes used inside this project:
+
+<table>
+  <thead>
+    <tr>
+      <th>Group</th>
+      <th>Scope</th>
+      <th>Definition</th>
+    </tr>
+  </thead>
+  <tbody>
+    <tr>
+      <td rowspan="9">Crates</td>
+      <td><code>sdk</code></td>
+      <td>About the <code>matrix-sdk</code> crate.</td>
+    </tr>
+    <tr>
+      <td><code>appservice</code></td>
+      <td>About the <code>matrix-sdk-appservice</code> crate.</td>
+    </tr>
+    <tr>
+      <td><code>base</code></td>
+      <td>About the <code>matrix-sdk-base</code> crate.</td>
+    </tr>
+    <tr>
+      <td><code>common</code></td>
+      <td>About the <code>matrix-sdk-common</code> crate.</td>
+    </tr>
+    <tr>
+      <td><code>indexeddb</code></td>
+      <td>About the <code>matrix-sdk-indexeddb</code> crate.</td>
+    </tr>
+    <tr>
+      <td><code>qrcode</code></td>
+      <td>About the <code>matrix-sdk-qrcode</code> crate.</td>
+    </tr>
+    <tr>
+      <td><code>sled</code></td>
+      <td>About the <code>matrix-sdk-sled</code> crate.</td>
+    </tr>
+    <tr>
+      <td><code>store-encryption</code></td>
+      <td>About the <code>matrix-sdk-store-encryption</code> crate.</td>
+    </tr>
+    <tr>
+      <td><code>test</code></td>
+      <td>About the <code>matrix-sdk-test</code> and <code>matrix-sdk-test-macros</code> crate.</td>
+    </tr>
+    <tr>
+      <td rowspan="4">Bindings</td>
+      <td><code>apple</code></td>
+      <td>About the <code>matrix-rust-components-swift</code> binding.</td>
+    </tr>
+    <tr>
+      <td><code>crypto-nodejs</code></td>
+      <td>About the <code>matrix-sdk-crypto-nodejs</code> binding.</td>
+    </tr>
+    <tr>
+      <td><code>crypto-js</code></td>
+      <td>About the <code>matrix-sdk-crypto-js</code> binding.</td>
+    </tr>
+    <tr>
+      <td><code>crypto-ffi</code></td>
+      <td>About the <code>matrix-sdk-crypto-ffi</code> binding.</td>
+    </tr>
+    <tr>
+      <td>Labs</td>
+      <td><code>sled-state-inspector</code></td>
+      <td>About the <code>sled-state-inspector</code> project.</td>
+    </tr>
+    <tr>
+      <td>Continuous Integration</td>
+      <td><code>xtask</code></td>
+      <td>About the <code>xtask</code> project.</td>
+    </tr>
+  </tbody>
+</table>
+
+## Generating `CHANGELOG.md`
+
+The [`git-cliff`](https://github.com/orhun/git-cliff) project is used
+to generate `CHANGELOG.md` automatically. Hence the various
+`cliff.toml` files that are present in this project, or the
+`package.metadata.git-cliff` sections in various `Cargo.toml` files.
+
+Its companion,
+[`git-cliff-action`](https://github.com/orhun/git-cliff-action)
+project, is used inside Github Action workflows.