RFR: 7903128: Overhaul jextract README [v5]

[Jorn Vernee]

On Thu, 24 Mar 2022 14:51:46 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 one additional commit since the last revision:
> 
>   Address review comments

README.md line 7:

> 5: ### Getting started
> 6: 
> 7: `jextract` depends on the [C libclang API](https://clang.llvm.org/doxygen/group__CINDEX.html). To build the jextract sources, the easiest option is to download LLVM binaries for your platform, which can be found [here](https://releases.llvm.org/download.html) (a version <= 9 is required). Both the `jextract` tool and the bindings it generates depend heavily on the [Foreign Function & Memory API](https://openjdk.java.net/jeps/419), so a suitable [jdk 18 distribution](https://jdk.java.net/18/) is also required.

should be `version >= 9` right?

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