Explain that setting -DMAXRANK=4 reduces compilation time

[Beliavsky]

Here is a minor suggestion for the Readme.md. It says

You can limit the maximum rank by setting -DMAXRANK= in the FYPPFLAGS environment variable:

make -f Makefile.manual FYPPFLAGS=-DMAXRANK=4

It may not be obvious to a new user why one would do this, but this drastically reduces the compilation time compared to

make -f Makefile.manual

So I suggest adding a sentence after the first command above: "Doing this substantially reduces compilation time."

[gareth-nx]

Hi @Beliavsky.

In case you'd consider using this issue to propose a simple pull request with a fix -- there is a tutorial on how to do that here.

My git skills are not yet sufficient to make proper contributions, but I followed that tutorial a few months ago to make a simple documentation fix, and found it relatively straightforward.

[milancurcic]

Fixed by #501.