Provide information in README how to build

[fogti]

Currently, it was a bit of a hassle to figure out how to build this project, from reading through

• the GitHub actions workflows
• scripts/buildga.sh
• make2/make.txt

To end up with:

git clone https://github.com/jsoftware/jsource
cd jsource
./make2/build_all.sh
./make2/cpbin.sh 
ln -s ../../profile.ijs ./bin/linux/*/

(the reason I was interested in this in the first place was https://chemnitzer.linux-tage.de/2024/en/programm/beitrag/289)

[bilam]

Information should already be provided in overview.txt

[fogti]

interestingly, overview.txt is pretty hard to read for me, I didn't understand it at first. make2/make.txt and such are pretty clear in contrast. also, why is overview.txt separate from README.md when it is so short?

[fogti]

I encountered other problems by now... I needed to pass -z noexecstack (see also) to libj's ld command line to ensure that the stack gets marked non-executable, and I found out I don't know how the libjpcre2.* files get generated/linked...