RFR: 8269244: [IDE] Dependency verification of *-sources.jar fails when doing gradle sync

[Marius Hanl]

This PR fixes the issue identified and discussed in PR https://github.com/openjdk/jfx/pull/506 which will make the gradle sync in IntelliJ fail because of the failing dependency verification for source files.

Gradle provides a way to skip the verification of source files, documented here:
https://docs.gradle.org/current/userguide/dependency_verification.html#sec:skipping-javadocs

Also the **README.txt** file is adjusted to include the instructions neccessary when updating the **verification-metadata.xml** file.

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Commit messages:
 - Fixed dependency verification of *-sources.jar fails when doing gradle sync

Changes: https://git.openjdk.java.net/jfx/pull/549/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=549&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269244
  Stats: 13 lines in 2 files changed: 12 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/549.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/549/head:pull/549

PR: https://git.openjdk.java.net/jfx/pull/549