Add a user friendly introduction to Muddle.

Some historical background, and a hello world example.

README.md

@@ -101,6 +101,7 @@ There are some short introductions for beginners:
 - [TECO survival guide](doc/TECO.md)
 - [DDT debugging newbie guide](doc/debugging.md)
 - [Hello MIDAS](doc/hello-midas.md)
+- [Introduction to Muddle](doc/muddle.md)
 - [DUMP and itstar](doc/DUMP-itstar.md)
 - [Games](doc/games.md)
 - [Printing](doc/printing.md)

doc/muddle.md

@@ -0,0 +1,76 @@
+# Introduction to Muddle
+
+### Background
+
+Muddle, later renamed MDL, was a programming language created by the
+Dynamic Modeling group (also known as the Programming Technology
+Division) at MIT.  The most famous application written in this
+language is the Zork interactive fiction game.  The influental CLU
+programming language was bootstrapped from Muddle.
+
+Muddle is a dialect of Lisp, but with the distinct difference that
+forms are bracketed by `<` and `>` instead of parentheses.  `(` and
+`)` still denote lists and are occasionally present in the language
+syntax.  The language is case sensitive, and all standard names must
+be entered in upper case.  To submit an expression to the interpreter,
+type the <kbd>Escape</kbd> key which echoes as `$`.
+
+### Example
+
+A common way to develop applications in Muddle, is to build a program
+in the interpreter and write out a `SAVE` image file.  This file can
+later be recalled using `RESTORE`.
+
+To make a "hello world" application, create a file called HELLO MUD
+with this content:
+
+```
+<DEFINE HELLO ()
+  <PRINC "Hello, muddled world!">
+  <TERPRI>
+  <QUIT>>
+
+<DEFINE SAVE-IT ()
+  <COND (<=? <SAVE "LARS; HELLO SAVE"> "SAVED"> <QUIT>)>
+  <HELLO>>
+
+<SAVE-IT>
+```
+
+`HELLO` is the main program which prints a message and terminates.
+`SAVE-IT` is used to create a `SAVE` file and arranges to call `HELLO`
+when it's restored.
+
+Now start `:muddle` and type `<FLOAD "HELLO MUD">` and
+<kbd>Escape</kbd>.  It should look like this:
+
+```
+:muddle
+MUDDLE 56 IN OPERATION.
+LISTENING-AT-LEVEL 1 PROCESS 1
+<FLOAD "HELLO MUD">$
+
+:KILL
+```
+
+A `HELLO SAVE` file has been created with the `HELLO` procedure in it.
+To run it, start `:muddle` again and type `<RESTORE "HELLO SAVE">`:
+
+```
+*:muddle
+MUDDLE 56 IN OPERATION.
+LISTENING-AT-LEVEL 1 PROCESS 1
+<RESTORE "HELLO SAVE">$
+Hello, muddled world!
+
+:KILL
+```
+
+### More Information
+
+For more information, see these documents:
+
+- [MDL Programming Primer](https://raw.githubusercontent.com/PDP-10/muddle/master/doc/MDL_Programming_Primer.pdf)
+- [MDL Programming Language](https://raw.githubusercontent.com/PDP-10/muddle/master/doc/MDL_Programming_Language.pdf)
+- [MDL Primer and Manual](https://raw.githubusercontent.com/PDP-10/muddle/master/doc/MDL_Primer_and_Manual.pdf)
+- [MDL Programming Environment](https://raw.githubusercontent.com/PDP-10/muddle/master/doc/MDL_Programming_Environment.pdf)