Fix README.md indentation

[landau]

While reading the docs I found some inconsistent indentation which made it more difficult to read. Here is the fix. I used 2 spaces because I saw some usage of it. Indentation was inconsistent in general. I find 2 spaces easier to read in READMEs

[rakyll]

In fact, continuation lines require 4 spaces. Other indentation should be represented with 2.