Add Visual Studio ignore rule for files generated by Microsoft Fakes.

Ignores generated Fakes assemblies and related files placed under the FakesAssemblies directory.
These files are generated at build time from the *.fakes configuration files.

VisualStudio.gitignore

@@ -139,6 +139,9 @@ App_Data/*.ldf
 *.bim.layout
 *.bim_*.settings
 
+# Microsoft Fakes
+FakesAssemblies/
+
 # =========================
 # Windows detritus
 # =========================