add ros2doctor README (#318)

* add README

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

* update sentence to new line

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

* Update grammer

Co-Authored-By: Marya Belanger <[email protected]>
Signed-off-by: claireyywang <[email protected]>

* Update grammer

Co-Authored-By: Marya Belanger <[email protected]>
Signed-off-by: claireyywang <[email protected]>

* Update grammer

Co-Authored-By: Marya Belanger <[email protected]>
Signed-off-by: claireyywang <[email protected]>

* Update grammar

Co-Authored-By: Marya Belanger <[email protected]>
Signed-off-by: claireyywang <[email protected]>

Author: Claire Wang

ros2doctor/README.md

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+# ros2doctor
+
+This folder contains the source code for ros2doctor.
+It is one of the ROS 2 command line interface tools included with a standard install of any ROS 2 distro.
+`ros2doctor` is similar to `roswtf` from ROS 1.
+It will examine your ROS 2 setup, such as distribution, platform, network interface, etc., and look for potential issues in a running ROS 2 system.
+
+## Usage
+
+Run `ros2 doctor` or `ros2 wtf`(alias) to conduct checks.
+
+Run `ros2 doctor -h/--help` to print all available command extensions.
+
+Run `ros2 doctor -r/--report` to see details of checks.
+
+## Add New Checks
+
+To add your own checks or information to report, use [Python entry points](https://setuptools.readthedocs.io/en/latest/pkg_resources.html#entry-points) to add modules to `setup.py`. See example below:
+
+```python
+    entry_points={
+        'ros2doctor.checks': [
+            'check_platform = ros2doctor.api.platform:check_platform',
+        ],
+        'ros2doctor.report': [
+            'report_platform = ros2doctor.api.platform:print_platform_info',
+            'report_ros_distro = ros2doctor.api.platform:print_ros2_info',
+        ],
+    }
+```