Remove info about marshaling of program

Currently this is not possible. Maybe in future such functionality will
be available. For now you should save original source and compile in before
running.

Author: antonmedv

docs/Usage.md

@@ -164,27 +164,6 @@ program, err := expr.Compile(`IsMore(City.Population)`, expr.Env(&EnvContextTwo{
 output, err := expr.Run(program, &EnvContextTwo{...})
 ```
 
-## Marshaling program
-
-Compiled program is possible to marshal and unmarshal before running.
-
-```go
-    env := map[string]int{
-		"foo": 1,
-		"bar": 2,
-	}
-
-	program, err := expr.Compile("foo + bar", expr.Env(env))
-	b, err := json.Marshal(program)
-
-	unmarshaledProgram := &vm.Program{}
-	err = json.Unmarshal(b, unmarshaledProgram)
-	
-	output, err := expr.Run(unmarshaledProgram, env)
-
-	fmt.Printf("%v", output) // outputs 3
-```
-
 ## Visitor
 
 [ast](https://godoc.org/github.com/antonmedv/expr/ast) package provides `Visitor` interface and `BaseVisitor` implementation.