Add a basic README in the api folder

Author: NlightNFotis

cpp_api/readme.md

@@ -0,0 +1,18 @@
+# The CPROVER C++ API
+
+This folder includes the interface and the implementation of a new C++-based
+API for the CProver libraries. The aim is for this to be able to be used by
+clients to drive all of the verification pipeline without the need to invoke
+any of the associated tools.
+
+## Implementation
+
+The major part of the implementation of the API is in [`api.cpp`](api.cpp).
+To use the API as it stands, you need to include the header [`api.h`](api.h)
+in your program, and link against the resultant shared-object from the CProver
+build process (`libcprover-api.a`).
+
+## Example
+
+An example for driving the API in its current form is in the file [`call_bmc.cpp`](call_bmc.cpp),
+which we are also using as a binary driver for testing the API.