Update pdq/cpp/README.md for facebook#868 (facebook#882)

Better links, remove legal advice

pdq/cpp/README.md

@@ -1,11 +1,11 @@
 # Documentation
 
-Please see `../README.md` and `../../README.md` for context, and `../../hashing.pdf` for thorough documentation.
+Please see [../README.md](https://github.com/facebook/ThreatExchange/blob/main/pdq/README.md) and [../../README.md](https://github.com/facebook/ThreatExchange/blob/main/README.md) for context, and [../hashing.pdf](https://github.com/facebook/ThreatExchange/blob/main/hashing/hashing.pdf) for thorough documentation.
 
 # Dependencies
 
 * C++ 11 or higher
-* `CImg.h` is included for reference, although note that its license does not allow commercial use. It is expected that your company will already have image-processing logic. Dependencies of this code on `CImg.h` are confined solely to `io/pdqio.h` and `io/pdqio.cpp` which you can customize for your site.
+* `CImg.h` is included for reference, though note it is not under the same license as the rest of the repository. It is expected that your company will already have image-processing logic. Dependencies of this code on `CImg.h` are confined solely to `io/pdqio.h` and `io/pdqio.cpp` which you can customize for your site.
 * ImageMagick or other JPEG/PNG libraries: for example, `brew install imagemagick` on MacOSX
 
 # Contact