Add run instructions to Readme.md about ortho4xp

[OldSubSailor]

At this time, these are listed in the install instructions text file:
Mac
4) Launch Ortho4XP
python3 Ortho4XP_v130.py

Windows
Open a command window in the Ortho4XP directory (freshly downloaded from Github)
and launch "python Ortho4XP_v130.py".

I don't see any for Linux, perhaps the Mac instructions?

With regard to Mac, I have been trying to get the Ortho script to be double click-able.
Alas, Git has said that the Python Launcher appears to be "broken" on Catalina. Perhaps someone with an earlier macOS could give it a test?

I can get a sort of work around by using IDLE as the default application and double click.

It does appear that it would be a simple copy and paste from install to Readme. But here it is, over two years . . .

[turowicz]

Thanks!

[OldSubSailor]

You are welcome.