[code-reflection] Integrated: Added TLDR build+run instructions to README and simplified hat.java

[Gary Frost]

Added a TLDR stanza to the README.md 

Also simplified the `hat.java` bld/run script.

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Commit messages:
 - Added TLDR  build+run instructions to README and simplified hat.java script

Changes: https://git.openjdk.org/babylon/pull/653/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=653&range=00
  Stats: 118 lines in 4 files changed: 49 ins; 3 del; 66 mod
  Patch: https://git.openjdk.org/babylon/pull/653.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/653/head:pull/653

PR: https://git.openjdk.org/babylon/pull/653