add doc

test=develop

paddle/fluid/imperative/README.md

@@ -91,7 +91,7 @@ class Tracer {
 ```
 
 * Trace forward operations
-* Perform simple python level infer and return to user.
+* Perform quick shape/type infer, push kernel execution engine and return to user.
 * Perform autograd to generate gradients.
 * Clear trace.
 * Apply gradients with optimizers
@@ -113,6 +113,20 @@ when needed.
 
 Lazy execution of pushed C++ operations.
 
+## Device Placement
+
+* Operator executes on the inputs' device.
+* All inputs should live on the same device.
+* use `Var.to()` to explicitly move var to a device.
+
+## Save/Load Models
+
+TODO
+
+## I/O
+
+TODO
+
 ## Refactor
 
 * All function layers with parameters converted to class Layers.
@@ -181,15 +195,6 @@ class MLP(fluid.imperative.Layer):
      out._backward()
 ```
 
-
-## Save/Load Models
-
-TODO
-
-## I/O
-
-TODO
-
 # Plan
 
 2.1，3 fulltime, Can run a few simple models. (Currently, 2 20% engs)