Add warning message on skipped migration files and document migration file format in tutorial

This change is based on proposal by Ernst Haagsman

doc/tutorial.md

@@ -19,6 +19,15 @@ foodb
 Every sql file has special operation on table `ops`.
 This will help in understanding what is going on in each pgmigrate run.
 
+## Migration file name pattern
+
+All migration files should have versions and
+names in the following format
+```
+V<version>__<description>.sql
+```
+Note: files not matching this pattern will be skipped.
+
 ## Creating `foo` user and `foodb`
 
 We'll need dummy user and database for our experiments.

pgmigrate.py

@@ -167,6 +167,9 @@ def _get_migrations_info_from_dir(base_dir):
             continue
         match = MIGRATION_FILE_RE.match(fname)
         if match is None:
+            LOG.warning(
+                'File %s does not match by pattern %s. Skipping it.',
+                file_path, MIGRATION_FILE_RE.pattern)
             continue
         version = int(match.group('version'))
         ret = dict(