add some documentations

docs/getting_started.md

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+# Getting started
+### Installation and End-to-End Examples
+Please refer to the main `README.md`
+
+### Recommended practice
+The default parameters do not always gaurantee satisfying reconstructions.
+Regarding this, there are several things which can generally help
+
+#### Share camera parameters as much as possible
+If images are known to be taken with the same camera, or images are well organized and known to be taken by several cameras, it is higly recommended to share the camera intrinsics
+To achieve this, one can set `--ImageReader.single_camera_per_folder` or `--ImageReader.single_camera_per_image` in `colmap feature_extractor` to be 1.
+
+#### Allow larger epipolar error
+If images are of high resolution, or are blurry, it is worth trying to increase the allowed epipolar error by modifying `--RelPoseEstimation.max_epipolar_error`. For example, make it 4, or 10.