RFR: Add third-party sample list to README, populate with java-vulkan.

[Michael Ennen]

I made a more complex example (1000 lines, mostly because setting up a boilerplate Vulkan program is super lengthy) of using jextract on Vulkan and Win32 APIs.

I think it may be too lengthy and too much of a maintenance burden to commit to the `/samples` directory, so I thought maybe including a section on third-party samples would be better.

If this is not something you want to entertain that's totally understandable, but thought I would put it out there a bigger sample that interfaces with a pretty complex API may be helpful to others.

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Commit messages:
 - Add third-party sample list to README, populate with java-vulkan.

Changes: https://git.openjdk.org/jextract/pull/93/files
 Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=93&range=00
  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jextract/pull/93.diff
  Fetch: git fetch https://git.openjdk.org/jextract pull/93/head:pull/93

PR: https://git.openjdk.org/jextract/pull/93