Update README.md

[mmodenesi]

When the quickstart suggests:

Some test topics have already been created by default and Burrow can be accessed on port 8000 of your docker-machine. e.g. http://192.168.99.100:8000/v2/kafka

a newcomer eager to quiclky get a grasp of the project will be disappointed to receive

{"error":true,"message":"invalid request type","result":{}}

Instead, it shoud say

Some test topics have already been created by default and Burrow can be accessed on port 8000 of your docker-machine. e.g. http://192.168.99.100:8000/v3/kafka

or simply link to the available endpoints.

[ratishr]

Commited updated README.md