Add a Porting Reminders section to the README

Author: mattico

README.md

@@ -24,6 +24,19 @@ See [rust-lang/rust#35437][0].
 [2]: https://github.com/rust-lang/compiler-rt/tree/8598065bd965d9713bfafb6c1e766d63a7b17b89/lib/builtins
 [3]: https://travis-ci.org/japaric/rustc-builtins
 
+### Porting Reminders
+
+1. [Rust][4] and [C][5] have slightly different operator precedence.
+2. C assumes wrapping operations everywhere. Rust panics on overflow when in debug mode. Consider using the [Wrapping][6] type or the explicit [wrapping_*][7] functions where applicable.
+3. Note [C implicit casts][8], especially integer promotion. Rust is much more explicit about casting, be sure that any cast which affects the output is ported to the Rust implementation.
+4. Rust has many functions for integer or floating point manipulation in the standard library. Consider using one of these functions rather than porting a new one. 
+
+[4]: https://doc.rust-lang.org/reference.html#operator-precedence
+[5]: http://en.cppreference.com/w/c/language/operator_precedence
+[6]: https://doc.rust-lang.org/core/num/struct.Wrapping.html
+[7]: https://doc.rust-lang.org/std/primitive.i32.html#method.wrapping_add
+[8]: http://en.cppreference.com/w/cpp/language/implicit_conversion
+
 ## Progress
 
 - [x] adddf3.c