core, main: make missing config file section behavior configurable

refs: #1499

Change-Id: I01cce3c73214f592d8c342d8aeda4fbafc6804b8
docs: Adding installation instructions and configuration tips

README.md and INSTALL.md now redirect to documentation in docs/ folder.
Later we will add links to HTML pages with NFD documentation.

Change-Id: I4f02f3166e7802570b24ed225ac4de8c56c6e64e
Refs: #1446
rib: Switching to use global IO

Change-Id: I5eca302c65d868810bd59ab654b7ea74762dc7ee