Update README.md for android kotlin callouts

[ColtonIdle]

Please DO NOT FORCE PUSH. Don't worry about messy history, it's easier to do code review if we can tell what happened after the review, and force pushing breaks that.

Please make sure that your PR allows edits from maintainers. Sometimes it's faster for us to just fix something than it is to describe how to fix it.

After creating the PR, please add a commit that adds a bullet-point under the [Unreleased] section of CHANGES.md, plugin-gradle/CHANGES.md, and plugin-maven/CHANGES.md which includes:

• a summary of the change
• either
  • a link to the issue you are resolving (for small changes)
  • a link to the PR you just created (for big changes likely to have discussion)

If your change only affects a build plugin, and not the lib, then you only need to update the plugin-foo/CHANGES.md for that plugin.

If your change affects lib in an end-user-visible way (fixing a bug, updating a version) then you need to update CHANGES.md for both the lib and all build plugins. Users of a build plugin shouldn't have to refer to lib to see changes that affect them.

This makes it easier for the maintainers to quickly release your changes :)

[Goooler]

Seems we can call out

spotless/plugin-gradle/README.md

Lines 210 to 216 in 34b8ef8

	The target is usually inferred automatically from the java source sets. However, Spotless cannot automatically detect [android](https://github.com/diffplug/spotless/issues/111) or [java-gradle-plugin](https://github.com/diffplug/spotless/issues/437) sources, but you can fix this easily:
	```gradle
	spotless {
	java {
	target 'src/*/java/**/*.java'
	```

as well.

[ColtonIdle]

Yeah I copied the code from there, but I suppose we can add a warning around it so that github md gives it that nice treatment. Also... does anyone know if both

target 'src//kotlin/**/.kt'
target 'src//java/**/.kt'

are needed? I'm assuming so... but idk.

[Goooler]

Yeah, need to add both src/kotlin/**/.kt and src/java/**/.kt if you mixed the using of two source sets.

[ColtonIdle]

Cool. FWIW. I thought src/kotlin/**/.kt was working for me (as my spotlessApply started to work reallllly fast)... but it wasn't actually working upon my files.

I started
#2437

which also led me to comment here
#111

All related to this readme PR 😄

If anyone can tell where I'm going wrong I'd appreciate it!