Add rate of change definition

doc/texinfo/overview.texi

@@ -74,6 +74,8 @@ The following well-known solidity interfaces are partially implemented in CIC na
 
 @include seal.sol.texi
 
+@include tokenratechange.sol.texi
+
 @include tokenvend.sol.texi
 
 @include tokenvote.sol.texi

doc/texinfo/tokenratechange.sol.texi

@@ -0,0 +1,19 @@
+@subsection TokenRateChange
+
+Enables detection of properties for CIC tokens that change value over time, e.g. through demurrage.
+
+It allows defining the granularity of the rate of change, aswell as a frequency for which the rate of change is applied.
+
+A method for canonical value change calculations is also provided.
+
+@table @dfn
+@item ERC165 Interface identifier
+@include ../../build/TokenRateChange.interface
+@item Solidity interface definition
+@include ../../build/contract_TokenRateChange.texi
+@item Reference implementations
+@itemize
+@item
+@uref{git://holbrook.no/erc20-demurrage-token.git,}
+@end itemize
+@end table