Rename current build directory

[mpilman]

#1534 is trying to resolve part of this - but this is not the correct solution for the problem.

Many tools/IDEs (and users) are building in $SOURCE_DIR/build. This is almost a standard way of building cmake projects.

So this directory should not exist. We should therefore rename our build directory so that we don't break the workflow of users and tools who do this. Another reason why this is bad is that there's a risk that someone could accidentally add a file to build that will then be overwritten by cmake.

After build was renamed we can add the whole build directory to .gitignore.

[PierreZ]

Now that #5162 is merged, we can:

• add the whole build directory to .gitignore,
• update README.md.

@AlvinMooreSr are you working on it? This seems like an easy PR for new contributors like me 😄