Add xcodegen documentation

[philip-segerfast]

I got this error when syncing my project with Gradle:

> Task :example:shared:podInstall FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':example:shared:podInstall'.
> 'pod install' command failed with code 1.
  Error message:

          Please, check that podfile contains following lines in header:
          source 'https://cdn.cocoapods.org'

          Please, check that each target depended on shared contains following dependencies:

I read in the kmp-xlog project that you need to run xcodegen for the sample project. After installing and running that it builds successfully.
Please add this to the documentation so that people like me who don't know much about iOS dev don't have to waste time on figuring this out!

Thank you!