The custom CMakeLists.txt in the port installs headers under `rply/` and
expects `#include <rply/rply.h>` correspondingly as the include.
However, the code samples in the RPly project use a plain `#include
"rply.h"` instead: https://w3.impa.br/~diego/software/rply/
This PR adds `include/rply/` to the `target_include_directories()` to
support both conventions.
The project exports only two distinctly-named headers (`rply.h` and
`rplyfile.h`) so the non-prefixed includes causing a collision somewhere
is unlikely.
Also added license info to vcpkg.json and a usage file.
