Improve installation instructions for macOS

See also this discussion from the escet-dev list:

• https://www.eclipse.org/lists/escet-dev/msg00043.html
• https://www.eclipse.org/lists/escet-dev/msg00044.html
• https://www.eclipse.org/lists/escet-dev/msg00045.html
• https://www.eclipse.org/lists/escet-dev/msg00050.html
• https://www.eclipse.org/lists/escet-dev/msg00051.html

It is noted that the current instructions are useable only for very old, pre Sierra macOS installations. They will thus need updating for Sierra, High Sierra, Mojave, Catalina and Big Sur versions of macOS. However, Eclipse 2020-06 only supports Mohave (10.14) and higher, so our instructions only need to cover those versions. It is unclear whether our milestone releases, which are now signed, actually need updated instructions.