Added instructions into README

README

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+How to use
+==========
+
+Populate directory with markdown files (they have to end with .markdown). You
+can also create arbitrarily nested subdirectories containing .markdown files.
+
+Sg will go recursively through all directories not beginning with an underscore
+("_") and convert the .markdown files into .html. The resulting HTML files will
+be saved into the _site directory, conserving the directory structure where the
+.markdown file was found.
+
+Put your static content into _static. Files and subdirectories will be copied
+into _site without any modifications.
+
+