RFR: Add checkout instructions to PR body

[Jorn Vernee]

Hi,

I'd like let the pr bot add the fetch comamnd found in the automatic emails to the PR body, so that people using only GitHub can see the instructions as well.

This adds a new "Instructions" section to the body of the PR message:

> ## Instructions
> To checkout these changes locally:
> ```bash
> $ git fetch https://git.openjdk.java.net/panama-foreign pull/16/head:pull/16
> $ git checkout pull/16
> ```

Testing:
- Running `.\gradlew test`

Any other recommendations for testing this?

Thanks,
Jorn

-------------

Commits:
 - 7de15f5c: Use instance method instead of passing PR as argument
 - 6f34266c: Add checkout instructions to PR body

Changes: https://git.openjdk.java.net/skara/pull/434/files
 Webrev: https://webrevs.openjdk.java.net/skara/434/webrev.00
  Stats: 16 lines in 1 file changed: 16 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/434.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/434/head:pull/434

PR: https://git.openjdk.java.net/skara/pull/434