Note on assignment ops

Closes #67

Author: nrc

guide/expressions.md

@@ -284,7 +284,8 @@ between a unary operator and its operand.
 
 ### Binary operations
 
-Do include spaces around binary ops (i.e., `x + 1`, not `x+1`) (including `=`).
+Do include spaces around binary ops (i.e., `x + 1`, not `x+1`) (including `=`
+and other assignment operators such as `+=` or `*=`).
 
 For comparison operators, because for `T op U`, `&T op &U` is also implemented:
 if you have `t: &T`, and `u: U`, prefer `*t op u` to `t op &u`. In general,
@@ -301,6 +302,9 @@ foo + bar + baz
     + qux + whatever
 ```
 
+Prefer line-breaking at an assignment operator (either `=` or `+=`, etc.) rather
+than at other binary operators.
+
 ### Control flow
 
 Do not include extraneous parentheses for `if` and `while` expressions.