Release 5.6.18

[tkralphs]

Release 5.6.18 has been made. For the diff from the last version, see here. A summary of the changes is as follows.

• Switch CI to Github Actions.
• Switch to automatically generated README.
• Update documentation to reflect new build procedures and use of coinbrew get latex2html conversion working again (see result here).
• Fix reported bugs (See SYMPHONY segfaults on a 1x1 matrix #171, gap_limit parameter query #177, Invalid reads identified in Valgrind #180).