Add a separate README.md for runtime

Author: abitrolly

grumpy-runtime-src/README.md

@@ -1 +1,3 @@
-../README.md
+This is home for Grumpy runtime. Programs produced by Grumpy depend on this Go library, which provides wrappers for system level functions like the CPython API.
+
+See [../README.md](../README.md) for details.