Improve README.md so that the markdown rendering is pretty and easy to read

Related to https://issues.apache.org/jira/browse/BIGTOP-1312 and https://issues.apache.org/jira/browse/BIGTOP-1383, README.md could use an overhaul.

If you're coming to this project from github (https://github.com/apache/bigtop), the first thing you see is the README rendered in markdown. But, it's not very easy to read.

There are several things which can be improved without changing the content:

• code formatting
• consistent indentation
• headings
• links
  Also, there are some minor errors that can be cleaned up as well.