Add script to generate user guide HTML

doc/user_guide.md

@@ -0,0 +1,19 @@
+#> Overview
+
+Propane is an LR Parser Generator (LPG) which:
+
+  * accepts LR(0), SLR, and LALR grammars
+  * generates a built-in lexer to tokenize input
+  * supports UTF-8 lexer inputs
+  * generates a table-driven parser to parse input in linear time
+  * is MIT-licensed
+  * is distributable as a standalone Ruby script
+
+${remove}
+WARNING: This user guide is meant to be preprocessed and rendered by a custom
+script.
+The markdown source file is not intended to be viewed directly and will not
+include all intended content.
+Visit [https://holtrop.github.io/rscons/index.html](https://holtrop.github.io/rscons/index.html)
+to view the properly rendered version.
+${/remove}