Added profile-defined extension text.

src/rv-profile-defined-extensions.adoc

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+== Profile-Defined Extensions
+
+This profile, as with earlier profiles, includes several new
+extensions defined directly in the profile text.  These
+profile-defined extensions name optional features or combinations of
+features that are already present in ratified specifications, but that
+were not previously explicitly named.  Once the profile is ratified,
+these extension definitions will move into the appropriate sections of
+the combined ISA manual. The combined ISA manual was not available at
+the start of this profile definition.  Future profile proposals will
+be presented as an update to the combined ISA manual, with new
+profile-defined extensions provided as edits to the appropriate
+sections of the combined ISA manual.