Convert README to markdown style.

Author: jgarzik

README

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+# UniValue
+
+## Summary
+
+A universal value class, with JSON encoding and decoding.
+
+UniValue is an abstract data type that may be a null, boolean, string,
+number, array container, or a key/value dictionary container, nested to
+an arbitrary depth.
+
+This class is aligned with the JSON standard, [RFC
+7159](https://tools.ietf.org/html/rfc7159.html).
+
+## Installation
+
+This project is a standard GNU
+[autotools](https://www.gnu.org/software/automake/manual/html_node/Autotools-Introduction.html)
+project.  Build and install instructions are available in the `INSTALL`
+file following the `autogen.sh` step.
+
+```$ ./autogen.sh
+$ ./configure
+$ make
+
+## Design
+
+UniValue provides a single dynamic RAII C++ object class,
+and minimizes template use (contra json_spirit).
+