Add introductory MuPDF documentation.

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+Basic MuPDF usage example
+=========================
+
+For an example of how to use MuPDF in the most basic way, see
+doc/example.c. To limit the complexity and give an easier introduction
+this code has no error handling at all, but any serious piece of code
+using MuPDF should use the error handling strategies described below.
+
+Common function arguments
+=========================
+
+Most functions in the MuPDF interface take an argument which is one of
+context, document, stream or device.
+
+A context contains global state used by MuPDF inside functions when
+parsing or rendering pages of the document. It contains e.g.:
+
+	 an exception stack (see error handling below),
+
+	 a memory allocator (allowing for custom allocators)
+
+	 a resource store (for caching of images, fonts, etc.)
+
+The other arguments -- document, stream and device -- all contain a
+reference to the context, in order to not require context to be passed
+to sets of functions that need to take a common argument for parsing
+or rendering reasons.
+
+Error handling
+==============
+
+MuPDF uses a set of exception handling macros to simplify error return
+and cleanup. Conceptually, they work a lot like C++'s try/catch
+system, but do not require any special compiler support.
+
+The basic formulation is as follows:
+
+	fz_try(ctx)
+	{
+		// Try to perform a task. Never 'return', 'goto' or
+		// 'longjmp' out of here. 'break' may be used to
+		// safely exit (just) the try block scope.
+	}
+	fz_always(ctx)
+	{
+		// Any code here is always executed, regardless of
+		// whether an exception was thrown within the try or
+		// not. Never 'return', 'goto' or longjmp out from
+		// here. 'break' may be used to safely exit (just) the
+		// always block scope.
+	}
+	fz_catch(ctx)
+	{
+		// This code is called (after any always block) only
+		// if something within the fz_try block (including any
+		// functions it called) threw an exception. The code
+		// here is expected to handle the exception (maybe
+		// record/report the error, cleanup any stray state
+		// etc) and can then either exit the block, or pass on
+		// the exception to a higher level (enclosing) fz_try
+		// block (using fz_throw, or fz_rethrow).
+	}
+
+The fz_always block is optional, and can safely be omitted.
+
+The macro based nature of this system has 3 main limitations:
+
+1)	Never return from within try (or 'goto' or longjmp out of it).
+	This upsets the internal housekeeping of the macros and will
+	cause problems later on. The code will detect such things
+	happening, but by then it is too late to give a helpful error
+	report as to where the original infraction occurred.
+
+2)	The fz_try(ctx) { ... } fz_always(ctx) { ... } fz_catch(ctx) { ... }
+	is not one atomic C statement. That is to say, if you do:
+
+		if (condition)
+			fz_try(ctx) { ... }
+			fz_catch(ctx) { ... }
+
+	then you will not get what you want. Use the following instead:
+
+		if (condition) {
+			fz_try(ctx) { ... }
+			fz_catch(ctx) { ... }
+		}
+
+3)	The macros are implemented using setjmp and longjmp, and so
+	the standard C restrictions on the use of those functions
+	apply to fz_try/fz_catch too. In particular, any "truly local"
+	variable that is set between the start of fz_try and something
+	in fz_try throwing an exception may become undefined as part
+	of the process of throwing that exception.
+
+	As a way of mitigating this problem, we provide an fz_var()
+	macro that tells the compiler to ensure that that variable is
+	not unset by the act of throwing the exception.
+
+A model piece of code using these macros then might be:
+
+	house build_house(plans *p)
+	{
+		material m = NULL;
+		walls w = NULL;
+		roof r = NULL;
+		house h = NULL;
+		tiles t = make_tiles();
+
+		fz_var(w);
+		fz_var(r);
+		fz_var(h);
+
+		fz_try(ctx)
+		{
+			fz_try(ctx)
+			{
+				m = make_bricks();
+			}
+			fz_catch(ctx)
+			{
+				// No bricks available, make do with straw?
+				m = make_straw();
+			}
+			w = make_walls(m, p);
+			r = make_roof(m, t);
+			// Note, NOT: return combine(w,r);
+			h = combine(w, r);
+		}
+		fz_always(ctx)
+		{
+			drop_walls(w);
+			drop_roof(r);
+			drop_material(m);
+			drop_tiles(t);
+		}
+		fz_catch(ctx)
+		{
+			fz_throw(ctx, "build_house failed");
+		}
+		return h;
+	}
+
+Things to note about this:
+
+a)	If make_tiles throws an exception, this will immediately be
+	handled by some higher level exception handler. If it
+	succeeds, t will be set before fz_try starts, so there is no
+	need to fz_var(t);
+
+b)	We try first off to make some bricks as our building material.
+	If this fails, we fall back to straw. If this fails, we'll end
+	up in the fz_catch, and the process will fail neatly.
+
+c)	We assume in this code that combine takes new reference to
+	both the walls and the roof it uses, and therefore that w and
+	r need to be cleaned up in all cases.
+
+d)	We assume the standard C convention that it is safe to destroy
+	NULL things.