add simple rust example

examples/rust/readme.txt

@@ -0,0 +1,25 @@
+A small example how to produce somewhat reasonably small MicroW8
+carts in rust.
+
+A nightly rust compiler is needed for the unstable sqrtf32
+intrinsic.
+
+Simply compiling with rustc as shown in build.sh results in a
+339 byte tunnel.wasm. Using wasm-opt this can be reduced to
+244 bytes.
+
+When you disassemble this wasm file using wasm2wat you can see
+these globals and exports:
+
+(global (;0;) i32 (i32.const 65536))
+(global (;1;) i32 (i32.const 65536))
+(export "__data_end" (global 0))
+(export "__heap_base" (global 1))
+
+They are meant to be used for heap allocations and stack for any
+values that are not simple scalars (i32, f32, etc.). Since our
+code doesn't actually use any of that, we can just delete them
+in a text editor and assemble the code again with wat2wasm.
+
+This gives us a 200 byte wasm file. Running this through
+uw8-tool pack brings us to the final size of 137 bytes.