README: Convert from `.txt` to `.rst` markup

Also add sections on licensing and contribution instructions.

README.rst

+KWSys
+*****
+
+Introduction
+============
+
+KWSys is the Kitware System Library.  It provides platform-independent
+APIs to many common system features that are implemented differently on
+every platform.  This library is intended to be shared among many
+projects at the source level, so it has a configurable namespace.
+Each project should configure KWSys to use a namespace unique to itself.
+See comments in `CMakeLists.txt`_ for details.
+
+.. _`CMakeLists.txt`: CMakeLists.txt
+
+License
+=======
+
+KWSys is distributed under the OSI-approved BSD 3-clause License.
+See `Copyright.txt`_ for details.
+
+.. _`Copyright.txt`: Copyright.txt
+
+Contributing
+============
+
+See `CONTRIBUTING.rst`_ for instructions to contribute.
+
+.. _`CONTRIBUTING.rst`: CONTRIBUTING.rst

README.txt

-KWSys provides a platform-independent API to many common system
-features that are implemented differently on every platform.  This
-library is intended to be shared among many projects, so it has a
-configurable namespace.  Each project should configure KWSys to use a
-namespace unique to itself.  See comments in CMakeLists.txt for
-details.
-
-You are probably reading this file in the source tree of a surrounding
-project.  In that case, see "../README.kwsys" for details of using
-KWSys in your project.
-
-See CONTRIBUTING.rst for instructions to contribute KWSys changes.