Make SDK naming better, update README.md

[hubcio]

The aim of this task is to change iggy folder to sdk and update README.md

README.md should be updated with:

• describe what's delivered via crates.io (maybe separate README.md for crates?)
• describe what's what in whole iggy git repo (folders) - eg. folder server is iggy-server binary and it's server - same for the rest of folders (in general: relation of package name, binary name and folder name in the repository)