[README] Add more information about getting UI tests running

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