RFR: Add section on pre-built binaries to README

[Athijegannathan Sundararajan]

On Fri, 7 Oct 2022 11:46:21 GMT, Athijegannathan Sundararajan <sundar at openjdk.org> wrote:

>> I've added a short section on where to find the pre-built binaries. I think building jextract is less important than the information on usage if people can get binaries, so I've moved the section on building and testing to the bottom.
>
> README.md line 7:
> 
>> 5: ### Getting jextract
>> 6: 
>> 7: Pre-built binaries for jextract are peridodically released here: https://jdk.java.net/jextract/. These binaries are built from the `master` branch of this repo, and generally target the foreign memory access and function API in the latest mainline JDK (for which binaries can also be found on [jdk.java.net](https://jdk.java.net)).
> 
> could  https://jdk.java.net/jextract/ be a link?

peridodically spelling

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PR: https://git.openjdk.org/jextract/pull/84