Documentation improvements

Writing docs for:

• I want to make my project work with Buildstream

• I want to build a project, what do I do?

• When I've finished building the project, what can I do with it? Post-build

• How can I get the output?

• Now I want to modify code and test the result Modifying and Testing

• How can I modify this module and test it against something which depends on it?

install buildstream