Add readme for examples

examples/README.md

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+# Example MiMa programs
+
+This folder contains a few example programs, both as `.mimasm` and as
+assembled `.mima` files.
+
+* [Basic programs](#basic-programs)
+* [Advanced programs](#advanced-programs)
+
+## Basic programs
+
+### [`jmp_to_address_in_acc.mimasm`](jmp_to_address_in_acc.mimasm)
+
+This program demonstrates two different techniques for jumping to an
+address that is currently stored in the `ACC` register.
+
+### [`call_ret.mimasm`](call_ret.mimasm)
+
+This program demonstrates how the `CALL` and `RET` instructions
+behave. It doesn't use any sort of stack, so the call depth is limited
+to 1.
+
+### [`call_ret_stack.mimasm`](call_ret_stack.mimasm)
+
+This program works similar to `call_ret.mimasm`, but uses the `SP`
+register for a stack. This way, it can have nested `CALL`s by storing
+the content of the `RA` register on the stack.
+
+## Advanced programs
+
+### [`stack.mimasm`](stack.mimasm)
+
+This program demonstrates the use of stack frames for calling a
+function and passing parameters.  To call a function, it creates a
+shared stack frame containing the function's input parameters and
+enough space for its return values.
+
+### [`fib.mimasm`](fib.mimasm)
+
+This program calculates the first few fibonacci numbers and stores
+them in consecutive memory locations. It uses a stack with stack
+frames and recursive calls according to the following pattern:
+
+``` c++
+int fib(int n) {
+    if (n == 0) return 0;
+    if (n == 1) return 1;
+    return fib(n - 1) + fib(n - 2);
+}
+```
+
+This recursive solution for calculating fibonacci numbers is by far
+not the most efficient, but it demonstrates recursion and stack usage
+quite well.