Update Readme to Include Other Linker Flags: -ObjC

[semireg]

I don't fully understand this fix, but in order for "private" source files (protected categories) to be included I had to add "Other Linker Flags: -ObjC" to the build settings to the dependent app. Without this I was receiving an "unrecognized selector sent to instance" for any category method. Cheers.

The place in the Readme should be https://github.com/jverkoey/iOS-Framework#first_parties at step 5/6.