Update the README instructions to clarify the importance of pulling submodules

[var-const]

No description provided.

[var-const]

Tested by doing a fresh clone running the git clone command provided and verifying that ./gradlew :firebase-firestore:check succeeds (whereas it fails for the previous iteration of the command). Let me know if there are any other targets I should check.

[google-oss-bot]

Coverage Report

Affected SDKs

• firebase-database

  SDK overall coverage changed from 51.18% (24a07db) to 51.14% (0fce68eb) by -0.05%.

  Filename	Base (24a07db)	Head (0fce68eb)	Diff
  WebsocketConnection.java	34.66%	32.39%	-2.27%

Test Logs

• Base (24a07db7): https://android-ci.firebaseopensource.com/view/gcs/android-ci/logs/postsubmit-check-coverage/1301156178344022018
• Head (0fce68eb): https://android-ci.firebaseopensource.com/view/gcs/android-ci/pr-logs/pull/firebase_firebase-android-sdk/1939/check-coverage-changed/1301678122424340483

Notes

HTML coverage reports can be produced locally with ./gradlew <product>:checkCoverage.
Report files are located at <product-build-dir>/reports/jacoco/.

Head commit (0fce68eb) is created by Prow via merging commits: 24a07db 22d5cb9.

[rlazo]

We don't use --remote-submodules when syncing because it pulls the submodules from "head" in their respective repos, not from the exact commit we want to.

Each project is responsible to update their remotes if deemed necessary.

[var-const]

Done, thanks!

[rlazo]

Nice catch, thanks!

[google-oss-bot]

@var-const: The following tests failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
copyright-check	d35ddf2	link	/test copyright-check
smoke-tests	d35ddf2	link	/test smoke-tests
check-changed	d35ddf2	link	/test check-changed

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