RFR: 7903128: Overhaul jextract README [v2]

[Athijegannathan Sundararajan]

On Thu, 24 Mar 2022 13:15:44 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> I've improved the README file, by putting the expanding the build section, and putting it more front and center.
>> I have also streamlined the remaining contents a little, and removed the parts that were describing the foreign API (not really needed here).
>
> Maurizio Cimadamore has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Simplify dir tree structure
>  - More fixes

LGTM

README.md line 15:

> 13: ```
> 14: 
> 15: After building, there should be a new `jextract` folder under `build` (the contents of this folder might vary depending on the platform):

Folder name is jextract.app on Mac. i.e., even folder name depends on platform

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Marked as reviewed by sundar (no project role).

PR: https://git.openjdk.java.net/jextract/pull/3