update dev version
minor fixes in fax page
sending fax does not support NAT
device synchronisation does not work from behind a NAPT
add general guidelines
add missing file...
Remove not relevant dahdi warning for fax detection
corrected availability values in agent status messages (cti protocol)
Merge branch 'guidelines'
add requirements.txt
fix flag for xivo-restapid
building XiVO Client on GNU/Linux requires Qt in $PATH
Add Yealink devices T20P
Add Yealink devices T26P
add missing fields in REST API devices doc
add options parameter to device REST API
update switchboard configuration
update developers documentation around provisioning plugins
remove reference to Aastra 6751i

The only plugin supporting it was the xivo-aastra-2.6 plugin, which has
been removed earlier this year.
move detailed upgrade notes into their own directories