Updated git ignore to keep osm_pbf in git but ignore pbf and state files. Updated osm_pbf readme

.gitignore

@@ -1,3 +1,4 @@
 .idea
 *.*~
-osm_pbf
+osm_pbf/*.pbf
+osm_pbf/*.state.txt

osm_pbf/readme.md

@@ -0,0 +1,4 @@
+This folder should contain a *.pbf and *.state.txt.
+
+The first pbf file encoutered will be imported, for this reason we recommend
+to only place one pbf in this folder so you can be sure which will be imported.