Added intro paragraph to C spec.
|# HPVM-C Language|
|# HPVM-C Language Specification|
|An HPVM program is a combination of host code and one or more data flow graphs (DFG) at the IR level. We provide C function declarations representing the HPVM intrinsics that allow creating, querying, and interacting with the DFGs. More details about the HPVM IR intrinsics can be found in [the HPVM IR Specification.](#hpmv-specification.md).|
|An HPVM-C program contains both the host and the DFG code. Each HPVM kernel, represented by a leaf node in the DFG, can be compiled to multiple different targets (e.g. CPU and GPU) as described below.|
|This document describes all the API calls that can be used in an HPVM-C program.|
|## Host API|