add implied bounds placeholder

Author: nikomatsakis

src/SUMMARY.md

@@ -25,6 +25,7 @@
     - [Lowering to logic](./traits-lowering-to-logic.md)
       - [Goals and clauses](./traits-goals-and-clauses.md)
       - [Equality and associated types](./traits-associated-types.md)
+      - [Implied bounds](./traits-implied-bounds.md)
       - [Region constraints](./traits-regions.md)
     - [Canonical queries](./traits-canonical-queries.md)
       - [Canonicalization](./traits-canonicalization.md)

src/traits-implied-bounds.md

@@ -0,0 +1,9 @@
+# Implied Bounds
+
+*to be written*
+
+Cover:
+
+- Why the `FromEnv` setup etc is the way it is
+- Perhaps move some of the material from 'lowering rules' in to here
+- Show various examples where you could go wrong