Add primitive readme

Signed-off-by: Matthew Ballance <matt_ballance@mentor.com>

Author: Matthew Ballance

README.md

@@ -1,2 +1,18 @@
 # scvpi
-Provides a stub implementation of (System)Verilog VPI functions for SystemC
+Provides a stub implementation of (System)Verilog VPI functions for SystemC. 
+Currently, this is just enough to allow cocotb to schedule coroutines. 
+Access to signals, and other RTL-like things is not currently implemented. The purpose
+of this package is to enable running a Python testbench with high-level SystemC
+models.
+
+
+## Using scvpi
+
+- Compile scvpi with your SystemC model
+    - Add <scvpi>/src/scvpi.cpp to your SystemC files
+    - Add <scvpi>/src as an include path
+    
+- Specify VPI libraries to load at runtime
+    - Add +vpi=<path_to_so> to the SystemC command line
+    
+