add installation instructions for OpenBSD

and document the possibility to use system boost
instead of the bundled version.
Fixing typo in OpenBSD install section.

Also added a note about OpenBSD not being officially supported.

Fixes #1263
Update documentation with FreeBSD installation instructions

@Valloric: I'm very happy to have YouCompleteMe with clang completion working 100% on FreeBSD 10.1 :) Thank you for this
FreeBSD installation, add system LD_LIBRARY_PATH

FreeBSD 10/11-CURRENT may not install without adding it.
Disable on large file

closes #1238
Ignoring the new "already being parsed" message

This exception comes from ycmd and is not something the user can do
anything about, so let's rather not show it.
Updating vimdoc from README