Updating simple_bridge related docs.

Recently I've added DefaultValue to simple_bridge' request functions: query_param and post_param,

```erlang
Req:query_param("test", 1),
Req:post_param("test", 1)
```

Both will result as 1 if "test" is not defined.

I've also added param/2 function, which returns Value|DefaultValue for given Key from request without care about POST/GET.

Now updating CB docs to reflect this  changes.