Merge branch 'reword-client-tutorial' of https://github.com/skip2/nghttp2 into skip2-reword-client-tutorial

The overall meaning of the tutorial is the same. I added text describing frame types where it helps explain the callbacks better, e.g. introducing the HEADERS frame before on_header_callback().

Also fixed a couple of typos:
- Fixed ``reacb`` typo to ``readcb``
- Added parens to some functions, e.g. ``on_frame_recv_callback`` => ``on_frame_recv_callback()``

The overall meaning of the tutorial is the same, except for a couple of changes:
- app_context struct was called "app_content", fixed the typo
- nghttp2_session_server_new2() is mentioned, but the example uses nghttp2_session_server_new() instead, changed the text to match