Readme.txt for creating c2pa-c binary.

- src
download/clone c2pa-c v0.12.0 and c2pa-rs v0.64.0 from github
https://github.com/contentauth

- changes
for c2pa-c
* change dependencies of c2pa-rs to local path
for c2pa-rs
* add Sony RAW and HEIF extensions
* add c2pa-c.dll properties by winresource

- build
build c2pa-c and c2pa-rs library
* win
  cd c2pa-rs
  gmake release
  cd c2pa-c
  gmake release  
* mac
  cd c2pa_c_ffi
  make release
  cd c2pa-c
  cmake --build . --config Release
