Add readme for Mono specific version of SPC (#1963)

* Add readme for Mono specific version of SPC

* Update src/mono/netcore/System.Private.CoreLib/README.md
Co-Authored-By: NStephen Toub <stoub@microsoft.com>

src/libraries/System.Private.CoreLib/src/README.md

@@ -2,9 +2,14 @@
 
 This directory contains the shared sources for System.Private.CoreLib library. It represents the majority of the CoreLib implementation.  Each flavor of the runtime (e.g. coreclr, mono) provides additional files as part of their build of CoreLib to complement this directory's contents.
 
-Runtime specific partial part which have shared part use runtime specific suffix to easy the navigation.
-    * `.CoreCLR.cs` for CoreCLR runtime
-    * `.Mono.cs` for Mono runtime
+The goal is to have the majority of code located in this folder, as that code is used by both Mono and CoreCLR runtimes. The source code can be shared as a whole file or at the member level by declaring a type as `partial` and having common parts stored here and the rest in runtime-specific location.
+
+### File Naming Convention
+
+Any runtime-specific `partial` part which also has a shared part sholud use a runtime-specific file name suffix to ease the navigation.
+
+* `*.CoreCLR.cs` for CoreCLR runtime
+* `*.Mono.cs` for Mono runtime
 
 ## System.Private.CoreLib CoreCLR Sources
 

src/mono/netcore/System.Private.CoreLib/README.md

+# System.Private.CoreLib for Mono Runtime
+
+This folder contains the Mono runtime-specific portion of System.Private.CoreLib.dll. See [/src/libraries/System.Private.CoreLib/src/](/src/libraries/System.Private.CoreLib/src/README.md) for more details.