Improve README with missing sections and fixes

Add API Documentation section explaining bundled docs in JARs, Building
section with mvn commands, common module to the Modules table, License
section, and fix Java version to match pom.xml.

Author: ViacheslavKlimov

README.md

@@ -9,6 +9,7 @@ Auto-generated Java REST clients for ThingsBoard, built from OpenAPI specificati
 | `ce` | `thingsboard-ce-client` | Community Edition REST client |
 | `pe` | `thingsboard-pe-client` | Professional Edition REST client |
 | `paas` | `thingsboard-paas-client` | Cloud (PaaS) REST client |
+| `common` | `thingsboard-client-common` | Handwritten shared code (not published separately) |
 
 All modules share the same `org.thingsboard.client` group ID and use the `org.thingsboard.client.api` / `org.thingsboard.client.model` packages.
 
@@ -61,6 +62,23 @@ ThingsboardClient client = ThingsboardClient.builder()
 client.login("tenant@thingsboard.org", "tenant");
 ```
 
+## API Documentation
+
+Each published JAR includes Markdown API documentation bundled under `api-docs/` in the classpath. This covers every controller endpoint (`*ControllerApi.md`) and every model class. To extract the docs from a dependency:
+
+```bash
+jar xf thingsboard-ce-client-4.4.0-SNAPSHOT.jar api-docs/
+```
+
+The same docs are also available in each edition's `docs/` directory in this repository.
+
+## Building
+
+```bash
+mvn compile                    # Build all modules
+mvn compile -pl ce             # Build CE only
+```
+
 ## Project Structure
 
 ```
@@ -77,7 +95,7 @@ thingsboard-java-client/
 │   ├── pom.xml
 │   ├── spec/openapi.json            # OpenAPI spec (committed)
 │   ├── src/main/java/               # Generated + common sources (committed)
-│   ├── docs/                        # Generated + common docs (committed)
+│   ├── docs/                        # Generated API docs + common docs (committed)
 │   └── target/generated/            # Raw generator output (gitignored)
 ├── pe/
 │   └── (same structure as ce)
@@ -147,3 +165,7 @@ Options:
 - `<edition>/docs/` (with `common/docs/` overlaid on top of generated docs)
 
 Output log: `generate-client.log` (overwritten on each run)
+
+## License
+
+This project is licensed under the [Apache License 2.0](https://www.apache.org/licenses/LICENSE-2.0).