Add cpr installation notices

README.md

@@ -24,9 +24,12 @@ InfluxDB C++ client library
  - C++17 compiler
 
 __Dependencies__
- - [cpr](https://github.com/libcpr/cpr) (required)
+ - [cpr](https://github.com/libcpr/cpr) (required)<sup>i)</sup>
  - boost 1.66+ (optional – see [Transports](#transports))
 
+ <sup>i)</sup> *cpr* needs to provide [CMake support](https://github.com/libcpr/cpr#find_package); some systems need to call `ldconfig` after *.so* installation.
+
+
 ### Generic
  ```bash
 mkdir build && cd build