Improve documentation of BigDecimal#sign

Fixes #78 by describing behaviour for positive and negative zero in the docs.

Author: mrzasa

ext/bigdecimal/bigdecimal.c

@@ -3606,8 +3606,10 @@ BigDecimal_limit(int argc, VALUE *argv, VALUE self)
 
 /* Returns the sign of the value.
  *
- * Returns a positive value if > 0, a negative value if < 0, and a
- * zero if == 0.
+ * Returns a positive value if > 0, a negative value if < 0.
+ * It behaves the same with zeros - 
+ * it returns a positive value for a positive zero (BigDecimal('0')) and
+ * a negative value for a negative zero (BigDecimal('-0')). 
  *
  * The specific value returned indicates the type and sign of the BigDecimal,
  * as follows: