Add simple nuget.config to avoid lots of package management warnings (#1301)

Otherwise, someone with multiple package sources defined gets dozens of
warnings like the following when building the repo:
```
warning NU1507: There are 4 package sources defined in your configuration. When using central package management, please map your packa
ge sources with package source mapping (https://aka.ms/nuget-package-source-mapping) or specify a single package source. The following sources are defined: ...
```

.gitignore

@@ -11,7 +11,6 @@ dotnet/.config
 *.user
 *.userosscache
 *.sln.docstates
-nuget.config
 
 # User-specific files (MonoDevelop/Xamarin Studio)
 *.userprefs

nuget.config

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<configuration>
+  
+  <packageSources>
+    <clear />
+    <add key="nuget.org" value="https://api.nuget.org/v3/index.json" />
+  </packageSources>
+
+  <packageSourceMapping>
+    <packageSource key="nuget.org">
+      <package pattern="*" />
+    </packageSource>
+  </packageSourceMapping>
+  
+</configuration>