Add nlohmann/json to list of indi-qhy dependencies

[cgobat]

I initially got this error when trying to cmake -DCMAKE_INSTALL_PREFIX=/usr . in indi-qhy:

CMake Error at CMakeLists.txt:20 (find_package):
  By not providing "Findnlohmann_json.cmake" in CMAKE_MODULE_PATH this
  project has asked CMake to find a package configuration file provided by
  "nlohmann_json", but CMake did not find one.

  Could not find a package configuration file provided by "nlohmann_json"
  with any of the following names:

    nlohmann_jsonConfig.cmake
    nlohmann_json-config.cmake

  Add the installation prefix of "nlohmann_json" to CMAKE_PREFIX_PATH or set
  "nlohmann_json_DIR" to a directory containing one of the above files.  If
  "nlohmann_json" provides a separate development package or SDK, be sure it
  has been installed.

After running a sudo apt install nlohmann-json3-dev, the above command worked just fine.

This PR adds this currently unmentioned dependency to the requirements list in the indi-qhy README file.

[cgobat]

I also wanted to check whether the version requirement of INDI≥1.9.6 stated here:

indi-3rdparty/indi-qhy/README

Line 9 in 04aa204

+ INDI >= v1.9.6 (http://www.indilib.org)

is still correct. I got some strange errors later on in the compilation process that made it seem like my main INDI library installation was out of date. I have v1.9.9 installed.

[cgobat]

Indeed, after uninstalling 1.9.9 and compiling/installing 2.1.0, the QHY driver builds just fine. So it would seem that the specified 1.9.6 minimum version is out of date. I can't say for sure what the minimum supported version actually is, but if I had to guess I would imagine it is 2.0.

If anyone knows the answer I'm happy to tack that change on to this PR as well.

[knro]

Right, everything should actually be updated to require INDI 2.1.0