Add README and CHANGELOG

README

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+libuecc is a very small generic-purpose Elliptic Curve Cryptography library
+compatible with Ed25519.
+
+Most documentation can be found as Doxygen comments in the ecc.h header
+file. You can use `make doxygen` after running CMake to create HTML
+documenation from it.
+
+There are two sets of functions converting between libuecc's internal point
+representation and coordinates or compressed representation. The functions
+ending with _ed25519 use the same representation as original Ed25519
+implementation and should be used by new software. The functions with the
+suffix _legacy are provided for compatiblity with libuecc version before
+v6.
+
+Ed25519 and the legacy representation are isomorphic, they use a Twisted
+Edwards Curve
+
+    ax^2 + y^2 = 1 + dx^2y^2
+
+over the prime field for p = 2^255 - 19.
+
+Ed25519 uses the parameters
+
+    a = -1 and
+    d = -(121665/121666),
+
+while the legacy curve has
+
+    a = 486664
+    d = 486660.