Add git submodules for specs

[anatoly-scherbakov]

I think my previous PR was trying to change too much and I decided it would be easier to recreate the most important changes in a new one.

This PR:

• Creates two submodules (as discussed, normalization which is now rdf-canon is out of scope for now),
• Wires them into CI (thanks @davidlehn for introducing GitHub Actions!)

• Include JSON-LD specifications as git submodules #181 Add submodules for JSON-LD API & Framing specifications
• Include JSON-LD specifications as git submodules #181 Use spec git submodules in CI

[mielvds]

I'm in favor if this approach because the advantages outweigh the disadvantages. If this ever changes, it's not that much work to change it.

@anatoly-scherbakov as a sort of compromise: should we add a 'testing without submodules' paragraph to the README.md? Basically what you said in #182 (comment) about changing the path of the specs.

[anatoly-scherbakov]

@mielvds sure; this PR, however, doesn't change the behavior of the test system. It only affects existence of submodules and usage of them in CI.

We can update the docs in another PR. Making PRs smaller could speed up merging.

[mielvds]

It kind of changes the test system, because contributors now need to init submodules instead of cloning them (although they still could). I'm all for small PR's, but I also like them to include documentation about the changes they introduce. It's a balance.
Also based on @davidlehn's concern in the previous PR:

Adding this feature forces the maintainers to learn to use it. Help us (me) out with a few more snippets of docs perhaps in the README and/or CONTRIBUTING. Maybe even more script/makefile targets to run. Specifically, what do we (I) need to type to keep the test suites synced up all the time.

So makefile is covered by #191, but I would add the snippets here because it's not that much work (I can add this).

[anatoly-scherbakov]

need to init submodules instead of cloning them

After this PR, contributors are not yet required to do so, they might continue cloning specs separately as before. This PR prepares the repo for a submodule-aware workflow.

In a follow up PR, I will:

• Update runtests.py to use submodules by default;
• Update documentation,

if that makes sense. Let me know what you think.

[mielvds]

allright with me! if possible, i'd like to wait for @davidlehn and @BigBlueHat for the final go?

[anatoly-scherbakov]

Thanks! I would also prefer the upstream, add-github-workflow to be merged first. On its merge, this PR will be automatically redirected to master and could be thereafter merged.