Add some doc about CLI option and better informative message

doc/0.getting_started.md

@@ -53,6 +53,9 @@ You can pass some command-line arguments to Flatisfy commands, common to all the
 * `--max-entries N` to overload the `max_entries` value from config.
 * `-v` to enable verbose output.
 * `-vv` to enable debug output.
+* `--constraints` to specify a list of constraints to use (e.g. to restrict
+  import to a subset of available constraints from the config). This list
+  should be passed as a comma-separated list.
 
 
 ## Configuration

flatisfy/config.py

@@ -231,7 +231,9 @@ def load_config(args=None):
     # Handle constraints filtering
     if args and getattr(args, "constraints", None) is not None:
         LOGGER.info(
-            "Filtering constraints from config according to CLI argument."
+            ("Filtering constraints from config according to CLI argument. "
+             "Using only the following constraints: %s."),
+            args.constraints.replace(",", ", ")
         )
         constraints_filter = args.constraints.split(",")
         config_data["constraints"] = {