Add min curl version to README

[mattjala]

For #67

Specifically, 7.61.0 is required due to the use of CURLAUTH_BEARER

[jwsblokland]

Would it be a good idea to replace

find_package (CURL REQUIRED)

by

find_package (CURL 7.61.0 REQUIRED)

in the CMakeLists.txt such CMake will report an error if the found cURL version is smaller than the required version.