Fix-up readme and add in workaround for newer .net core sdk (#473)

Author: jamesmontemagno

Xamarin.Essentials/Xamarin.Essentials.csproj

@@ -90,6 +90,9 @@
     <Reference Include="System.Numerics" />
     <Reference Include="System.Numerics.Vectors" />
   </ItemGroup>
+  <ItemGroup>
+    <None Include="**\*.cs" Exclude="$(DefaultItemExcludes);$(DefaultExcludesInProjectFolder)" />
+  </ItemGroup>
   <Import Project="$(MSBuildSDKExtrasTargets)" Condition="Exists('$(MSBuildSDKExtrasTargets)')" />
   <Import Project="$(MSBuildThisFileDirectory)mdoc.targets" />
   <Import Project="$(MSBuildThisFileDirectory)..\CodeStyles.targets" />

nugetreadme.txt

@@ -10,37 +10,10 @@ Ensure that you install Xamarin.Essentials into all of your projects.
 For Android projects there is a small amount of setup needed to handle permissions. Please follow our short guide at:
 http://aka.ms/essentials-getstarted
 
-
-## Changes for 0.9.1-preview
-
-If you are upgrading from an earlier version there are a few changes to the API that may affect your code:
-
-* Android: VersionTracking has been reset
-* SensorSpeed.Ui is now SensorSpeed.UI
-* BatteryPowerSource.Ac is now BatteryPowerSource.AC
-* Change to generic EventHandlers for Accelerometer, Battery, Compass, Connectivity, Display Metrics, Magnetometer, OrientationSensor, Gyroscope, and Power.
-* BrowserLaunchType is now BrowserLaunchMode
-
-You may have registered an event handler for one of these:
-
-Gyroscope.ReadingChanged += OnReadingChanged;
-
-and then implemented the handler:
-
-void OnReadingChanged(GyroscopeChangedEventArgs e)
-{
-}
-
-Instead of using a custom EventHandler we now use EventHandler<T> which always pass in an object as the first parameter:
-
-void OnReadingChanged(object sender, GyroscopeChangedEventArgs e)
-{
-}
-
+## Release Notes
 
 See our full release notes: http://aka.ms/essentials-releasenotes
 
-
 ## Feedback, Issues, & Feature Requests
 
 We would love to hear from you simply head to: http://aka.ms/essentialsfeedback