Update imfile.rst to state readmode=2 support SP and TAB
Update imfile.rst to state readmode=2 support SP and TAB

As discussed in http://lists.adiscon.net/pipermail/rsyslog/2016-October/043279.html, and after code review https://github.com/rsyslog/rsyslog/blob/master/runtime/stream.c#L815

janmejay/rsyslog-doc,janmejay/rsyslog-doc,janmejay/rsyslog-doc,janmejay/rsyslog-doc