Document using DOCC_EXEC in SwiftPM

[natikgadzhi]

Bug/issue #, if applicable: N/A

Summary

This pull request adds a paragraph in README describing how to invoke a locally built docc when using SwiftPM, setting DOCC_EXEC env variable.

I was working on #652 and debugged my ideas that way, thought I'd add a doc entry for that.

Dependencies

None.

Testing

To test whether it works:

1. Build docc as usual
2. run DOCC_EXEC=./build/debug/docc swift generate-documentation --target=TARGET

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• [-] Added tests
• [-] Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[Kyle-Ye]

Feel a little weird for me. IMO, swift-docc developer should make direct call to docc instead of relying on swift-docc-plugin.

eg. docc convert/preview xx --additional-symbol-graph-dir xx

[natikgadzhi]

I do not disagree! But, felt that if xcodebuild behavior is documented, why not make it explicitly documented for the package manager plugin, too.

[natikgadzhi]

I don't like people who walk into a repo and do a "Hey you had some trailing space!", but I am that person now, it seems.

1. Cleaned up the langage, thank you to @Kyle-Ye for noticing a typo. It's in this commit so it's easier to review: 1d61565
2. A separate commit with formatting cleanup: 6e31c34 — I noticed my editor apply that, so I did a partial commit with an actually useful documentation piece separately, and then committed the formatting to, while I'm there.

[Kyle-Ye]

I don't like people who walk into a repo and do a "Hey you had some trailing space!", but I am that person now, it seems.

The "trailing space" is here on purpose. For some linter rule or Apple's writing guidance, we break the whole sentence into "2" lines(Line 3 and Line 4). But on the markdown side, it was still rendered as one line since there is no Hardbreak.

[natikgadzhi]

Thank you for pointing it out — I didn't know about that. I've removed the whole formatting commit — it feels like I don't know enough about internal systems to make it warranted and safe, and I don't feel there's enough value for beginner engineers reading the docs, or for the core team, to justify spending your time on reviewing it further.

[ethan-kusters]

Looks great! Thank you for making this change – I rely on this workflow of testing DocC via the SwiftPM plugin a lot and I think other contributors will appreciate having this documented.

I left some suggestions on wording/formatting but nothing blocking. Let me know what you think.

[ethan-kusters]

What do you think of avoiding the CLI abbreviation here? I think it's possible some readers won't be familiar with it.

Suggested change

	You can also test a locally built version of Swift-DocC using Swift Package Manager
	CLI. SwiftPM will try to read `DOCC_EXEC` environment variable value, and use
	You can also test a locally built version of Swift-DocC using the Swift Package Manager from the command line.
	SwiftPM will try to read `DOCC_EXEC` environment variable value, and use

[natikgadzhi]

@ethan-kusters, thank you for the review, and for the recommendations! Cleaned things up, should be ready to merge now.

[natikgadzhi]

Huh, TIL if I rebase the branch in GitHub UI, it removes the signature from my commits ;(

[ethan-kusters]

Looks great. Thank you!

[ethan-kusters]

@swift-ci please test