[Enhance]: Add documentation of result_analysis.py (open-mmlab#4642)

* Add result_analysis docs

* Update docs

Author: hhaAndroid

docs/useful_tools.md

@@ -48,6 +48,69 @@ Examples:
     average iter time: 1.1959 s/iter
     ```
 
+## Result Analysis
+
+`tools/analysis_tools/analyze_results.py` calculates single image mAP and saves or shows the topk images with the highest and lowest scores based on prediction results.
+
+**Usage**
+
+```shell
+python tools/analysis_tools/analyze_results.py \
+      ${CONFIG} \
+      ${PREDICTION_PATH} \
+      ${SHOW_DIR} \
+      [--show] \
+      [--wait-time ${WAIT_TIME}] \
+      [--topk ${TOPK}] \
+      [--show-score-thr ${SHOW_SCORE_THR}] \
+      [--cfg-options ${CFG_OPTIONS}]
+```
+
+Description of all arguments:
+
+- `config` : The path of a model config file.
+- `prediction_path`:  Output result file in pickle format from `tools/test.py`
+- `show_dir`: Directory where painted GT and detection images will be saved
+- `--show`：Determines whether to show painted images, If not specified, it will be set to `False`
+- `--wait-time`: The interval of show (s), 0 is block
+- `--topk`: The number of saved images that have the highest and lowest `topk` scores after sorting. If not specified, it will be set to `20`.
+- `--show-score-thr`:  Show score threshold. If not specified, it will be set to `0`.
+- `--cfg-options`: If specified, the key-value pair optional cfg will be merged into config file
+
+**Examples**:
+
+Assume that you have got result file in pickle format from `tools/test.py`  in the path './result.pkl'.
+
+1. Test Faster R-CNN and visualize the results, save images to the directory `results/`
+
+```shell
+python tools/analysis_tools/analyze_results.py \
+       configs/faster_rcnn/faster_rcnn_r50_fpn_1x_coco.py \
+       result.pkl \
+       results \
+       --show
+```
+
+2. Test Faster R-CNN and specified topk to 50, save images to the directory `results/`
+
+```shell
+python tools/analysis_tools/analyze_results.py \
+       configs/faster_rcnn/faster_rcnn_r50_fpn_1x_coco.py \
+       result.pkl \
+       results \
+       --topk 50
+```
+
+3. If you want to filter the low score prediction results, you can specify the `show_score_the` parameter
+
+```shell
+python tools/analysis_tools/analyze_results.py \
+       configs/faster_rcnn/faster_rcnn_r50_fpn_1x_coco.py \
+       result.pkl \
+       results \
+       --show-score-thr 0.3
+```
+
 ## Visualization
 
 ### Visualize Datasets