Improve apidoc with pydoctor tool (#28)

Using the [numpy doc style](https://numpydoc.readthedocs.io/en/latest/format.html). This format is also compatible with `pdoc` by specifying `--docformat=numpy`.

Author: tisonkun

README.md

@@ -110,3 +110,17 @@ Run a single unit test (e.g. `PulsarTest.test_tls_auth`):
 ```bash
 python3 ./tests/pulsar_test.py 'PulsarTest.test_tls_auth'
 ```
+
+## Generate API docs
+
+Pulsar Python Client uses [pydoctor](https://github.com/twisted/pydoctor) to generate API docs. To generate by yourself, run the following command in the root path of this repository:
+
+```bash
+pip3 install pydoctor
+pydoctor --make-html \
+  --html-viewsource-base=https://github.com/apache/pulsar-client-python/tree/<release-version-tag> \
+  --docformat=numpy --theme=readthedocs \
+  --intersphinx=https://docs.python.org/3/objects.inv \
+  --html-output=<path-to-apidocs> \
+  pulsar
+```