Add README to repository with minimal build/run instructions

Neither Okular repository nor website contain any information on how to build and run Okular from source. I figured out that it can be built via cmake --build . from repository root. But it isn't clear how to run ./bin/okular so that it loads plugins from <repo>/bin/plugins/okular instead of loading them from host system.

Instructions on running clang-format/clang-tidy would also be welcome.