Mention JDK requirements in README

[Marcono1234]

Before this PR

The class files of this project are compiled with Java 17 as target version, but it is currently not documented.

After this PR

The minimum JDK version is documented

Not sure if for the generated code a minimum JDK version can be specified as well. It probably mostly depends on what builder methods users are calling. For example if they use TypeSpec#recordBuilder, then the generated code can not be compiled with JDK 8.

[palantirtech]

Thanks for your interest in palantir/javapoet, @Marcono1234! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

pending cla

[Marcono1234]

pending cla

I had hoped a CLA will not be necessary. I will probably sign it though, but I have some questions which I wrote to [email protected] a few days ago, and will wait for an answer first.

[carterkozak]

Thanks! Let me know if you don’t hear back by next week, I can make sure things move along. I’m ooto until next Thursday, so my response time may be a bit slow in the meantime.

[Marcono1234]

It seems I haven't gotten any response yet. Maybe the e-mail address I had sent my questions from was a bit unusual, but the e-mail subject and body shouldn't have triggered any spam detection on Palantir's side I hope.