docs: make a more salient link to API docs (#274)

This PR is a small suggestion to add a salient link to the API docs for the package near the top of the readme. This may help with the discoverability of this important information.

Author: rickspencer3

README.rst

@@ -38,6 +38,8 @@ influxdb-client-python
 
 The API of the **influxdb-client-python** is not the backwards-compatible with the old one - **influxdb-python**.
 
+The full API reference for this package is available here: `https://influxdb-client.readthedocs.io/en/latest/api.html <https://influxdb-client.readthedocs.io/en/latest/api.html>`_
+
 InfluxDB 2.0 client features
 ----------------------------