Improvement to "Mandatory fields with @Builder" wiki page

[mernst]

I have an edit to
https://github.com/projectlombok/lombok/wiki/FEATURE-IDEA:-%22Mandatory%22-fields-with-@Builder
that documents the Called Methods Checker, which implements the third idea as a compiler plugin.
My edits appear in branch mandatory-fields-implementation of https://github.com/mernst/lombok.wiki. (Due to a GitHub limitation, I cannot make a pull request, and you cannot browse to https://github.com/mernst/lombok.wiki, but you can git clone it.)

Could you please review my edits and merge them into the main wiki repository at https://github.com/projectlombok/lombok.wiki?