add meta.yml and generate README and CI configuration from templates

[palmskog]

There is not really much point in testing a pure Coq library with several different versions of OCaml. This simplifies CI configuration to only test with each Coq version, and adds the usual badges, etc., to the README.

[palmskog]

@KevOrr do you mind if I merge this?

[KevOrr]

@palmskog sorry for the extremely late reply. I'm not sure how I never saw this PR until I got the notification it was merged.

Thank you for the changes, this really does simplify things. The OPAMWITHTEST environment variable is very helpful to avoid having to write a custom_script for docker-coq-action.

One question I have is what the effect of + in the recursive make invocations is. Afaict, the + only seems to have an effect if make is run with the -n flag, in which case it will only run the lines which have the + flag. Is that accurate?

Edit: ah, is the idea to recursively invoke make, but also run it with -n, so that all the commands that would have been run during a normal invocation would be printed, even if they reside in child Makefiles?

[palmskog]

@KevOrr the + stuff has to do with makefile parallelism, specifically to ensure we don't get this dreaded message with, say, 10-way parallelism:

warning: jobserver unavailable: using -j1. 

You can read more about it on StackOverflow.

[KevOrr]

Thanks for the reference. I might be misreading, but it seems from that SO answer and the GNU Make docs that it links, that using the MAKE variable directly already tells Make that it's launching a child Make process and to set up the necessary IPC. But it doesn't cause any harm and if MAKE does not appear directly in the recipe but only indirectly through variable expansion, then the + becomes necessary to set up IPC. So I can see how it'd be a good habit to get into for any recursive invocation.