Convert README to markdown

This is the format used most often on GitHub when introducing a software
package to potential users and developers, and allows for simple formatting
of the README document.  As part of this change I added headers for
installation and documentation sections.

README.md

@@ -0,0 +1,23 @@
+# Test::MockModule - mock subroutines in a module
+
+See the LICENSE section in `lib/Test/MockModule.pm` for usage and
+distribution rights.
+
+## Installation
+
+Run these commands in the source directory:
+
+    perl Build.PL
+    ./Build
+    ./Build test
+    ./Build install
+
+Then delete the source directory tree since it's no longer needed.
+
+Alternatively, you can simply install the module via `cpanm`:
+
+    cpanm Test::MockModule
+
+## Documentation
+
+Run `perldoc Test::MockModule` to read the full documentation.