Describe cmake dependency in README
Describe cmake dependency in README

If and when syntect switches to fancy-regex from onig, then we can
take this back out :)

Fixes the problem raised in #127

modelorganism/xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,fuchsia-mirror/third_party-xi-editor,google/xi-editor,google/xi-editor,google/xi-editor,google/xi-editor,fuchsia-mirror/third_party-xi-editor,modelorganism/xi-editor,modelorganism/xi-editor