init: Update readme.txt to reflect recent changes to init.

Updates are as follows:

A detailed explanation of event triggers and property triggers
including how they can now be used

A new section describing .rc files, their locations, and the
intentions for each of these locations

A new section describing the import keyword and how it is not a
command but rather its own section

Removal of deprecated or incorrect text

Change-Id: If0a37375ac92a857900f3303ada4ba742360daff