Added CommCare External Instances section, closes https://github.com/dimagi/xform-spec/issues/3
bindings - predicates, closes https://github.com/dimagi/xform-spec/issues/5
bindings - XPath functions removed/added for CommCare, closes https://github.com/dimagi/xform-spec/issues/2
removed acknowledgements
edited structure
styling tweak
edited intro
bindings and body: removed unsupported geotrace and geoshape