Xamarin.forms: [README] Add more information about getting UI tests running

[samhouts]

I have to say. It took a while before my environment was ready. There are some things missing in the "getting started" section. Especially for running UITest.

First problem was that the test weren't discovered.

• Either install older version of nunit test adapter extension
• Or change a setting in Visual Studio
  I changed the setting in Visual Studio and it worked (due to the fact the correct nunit nuget packages were already added to the project).

Then came another problem while running the UITest.
It complained there was no ANDROID_HOME environment variable.
Fixed that by adding it.

Then when running the UITest, it complained about not finding the adb. Something todo with mismatching sdk versions. After a quick google I found this: Getting your xamarin uitests to actually work. So I downgraded the platform tools and it finally worked.
If that didn't worked, I would have just given up. It was just not worth the hassle.

If all those things were properly documented as possible pittfalls and available in the "getting started" section, it would help alot of starting contributers like me.

_Originally posted by @puppetSpace in https://github.com/xamarin/Xamarin.Forms/issues/2381#issuecomment-481236329_

[techduggu]

I'm also facing the above issues while running the UI Tests. At least we can start with a draft template and then can continue adding our observations). I'm working on Windows machine and can only help from Android side (would need help from someone for iOS).
I also see that we have tagged it with 'hacktoberfest' tag, Is this considered as quality PR? If yes, I'd like to take this up.

[ajpinedam]

@techduggu I can help with the iOS side