Adds additional information for Windows users.

A lot of trouble that windows users seem to have is that the engine API works differently on Windows.

According to Mathworks docs:
Instead of launching a new process (like on Mac/Linux) it opens a COM channel to the version of MATLAB that is registered in Windows. If multiple MATLAB versions are used or MATLAB was updated, it can be that the version Julia uses is not the same that is registered in Windows.

There are two options:

1. Use version that is registered (rebuild MATLAB.jl) - but its unclear to find out which version is registered.
2. Open a specific MATLAB version and manually register it. Then adapt MATLAB.jl (if needed)

Until we can do option 1 automatically, I think for users, its easier to perform option 2. I added the instructions into the warning that is printed.

The old instruction suggested doing this in an command prompt but additional information were missing how to control the version that is actually getting registered. To make it smoother and avoid confusion, I think the graphical way is better suited and I removed the mention of the CLI way.

Closes #167

Related: #182

Relevant docs from Mathworks:

• Behaviour on Windows: https://de.mathworks.com/help/matlab/apiref/engopen.html
• Manually register: https://de.mathworks.com/help/matlab/matlab_external/registering-matlab-software-as-a-com-server.html