Update guidelines.md (#2095)

Author: kailuowang

docs/src/main/tut/guidelines.md

@@ -69,7 +69,7 @@ The user doesn't need to specify the type `A` which is given by the parameter.
 You probably noticed that there is a `val dummy: Boolean` in the `PurePartiallyApplied` class. This is a trick we used
 to make this intermediate class a [Value Class](http://docs.scala-lang.org/overviews/core/value-classes.html) so that there is no cost of allocation, i.e. at runtime, it doesn't create an instance of `PurePartiallyApplied`. We also hide this partially applied class by making it package private and placing it inside an object.
 
-### <a id="ops-classes" href="#implicit-naming"></a> Implicit naming
+### <a id="implicit-naming" href="#implicit-naming"></a> Implicit naming
 
 In a widely-used library it's important to minimize the chance that the names of implicits will be used by others and
 therefore name our implicits according to the following rules:
@@ -84,5 +84,32 @@ As an example, an implicit instance of `Monoid` for `List` defined in the packag
 
 This rule is relatively flexible. Use what you see appropriate. The goal is to maintain uniqueness and avoid conflicts.
 
+
+
+### <a id="implicit-priority" href="#implicit-priority"></a> Implicit instance priority
+
+When there are multiple instances provided implicitly, if the type class of them are in the same inheritance hierarchy, 
+the instances need to be separated out into different abstract class/traits so that they don't conflict with each other. The names of these abstract classes/traits should be numbered with a priority with 0 being the highest priority. The abstract classes/trait
+with higher priority inherits from the ones with lower priority. The most specific (whose type class is the lowest in the hierarchy) instance should be placed in the abstract class/ trait with the highest priority.  Here is an example. 
+
+```scala
+@typeclass 
+trait Functor[F[_]]
+
+@typeclass
+trait Monad[F[_]] extends Functor
+
+...
+object Kleisli extends KleisliInstance0 
+
+abstract class KleisliInstance0 extends KleisliInstance1 {
+  implicit def catsDataMonadForKleisli[F[_], A]: Monad[Kleisli[F, A, ?]] = ...
+}
+
+abstract class KleisliInstance1 {
+  implicit def catsDataFunctorForKleisli[F[_], A]: Functor[Kleisli[F, A, ?]] = ...
+}
+```
+
 #### TODO:
 Once we drop 2.10 support, AnyVal-extending class constructor parameters can be marked as private.