add README

Signed-off-by: mitukou1109 <[email protected]>

planning/autoware_path_generator/README.md

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+# Path Generator
+
+The `path_generator` node receives a route from `mission_planner` and converts the centerline into a path.
+If the route has waypoints set, it generates a path passing through them.
+
+This package is a simple alternative of `behavior_path_generator`.
+
+## Path generation
+
+When input data is ready, it first searches for the lanelet closest to the vehicle.
+If found, it gets the lanelets within a distance of `backward_path_length` behind and `forward_path_length` in front.
+Their centerlines are concatenated to generate a path.
+
+If waypoints exist in the route, it replaces the overlapped segment of the centerline with them.
+The overlap interval is determined as shown in the following figure.
+
+![waypoint_group_overlap_interval_determination](./media/waypoint_group_overlap_interval_determination.drawio.svg)
+
+## Input topics
+
+| Name                 | Type                                        | Description                      |
+| :------------------- | :------------------------------------------ | :------------------------------- |
+| `~/input/odometry`   | `nav_msgs::msg::Odometry`                   | ego pose                         |
+| `~/input/vector_map` | `autoware_map_msgs::msg::LaneletMapBin`     | vector map information           |
+| `~/input/route`      | `autoware_planning_msgs::msg::LaneletRoute` | current route from start to goal |
+
+## Output topics
+
+| Name            | Type                                       | Description    | QoS Durability |
+| :-------------- | :----------------------------------------- | :------------- | :------------- |
+| `~/output/path` | `tier4_planning_msgs::msg::PathWithLaneId` | generated path | `volatile`     |
+
+## Parameters
+
+| Name                                   | Type     | Description                                                                                                           |
+| :------------------------------------- | :------- | :-------------------------------------------------------------------------------------------------------------------- |
+| `planning_hz`                          | double | planning frequency                                                                                                    |
+| `backward_path_length`                 | double | length of the generated path behind vehicle                                                                           |
+| `forward_path_length`                  | double | length of the generated path in front of vehicle                                                                      |
+| `waypoint_group_separation_threshold`  | double | maximum distance at which consecutive waypoints are considered to belong to the same group (see [here](#path-generation) for details) |
+| `waypoint_group_interval_margin_ratio` | double | ratio for determining length of switching section from centerline to waypoints (see [here](#path-generation) for details)             |