[Test] Instruction for write regression test (#1341)

* patch

* fix+1

* add readme

tests/regression/README.md

+How to add test to regression
+=================================
+
+Official link to [asv](https://asv.readthedocs.io/en/stable/writing_benchmarks.html)
+
+
+## Add test
+
+DGL reuses the ci docker image for the regression test. There are four conda envs, base, mxnet-ci, pytorch-ci, and tensorflow-ci.
+
+The basic use is execute a script, and get the needed results out of the printed results.
+
+- Create a new file in the tests/regression/
+- Follow the example `bench_gcn.py` or the [official instruction](https://asv.readthedocs.io/en/stable/writing_benchmarks.html)
+  - function name starts with `track` will be used to generate the stats, by the return value
+  - setup function would be execute every time before running track function
+  - Can use params to pass parameter into `setup` and `track_` functions
+
+## Run locally
+
+```bash
+docker run --name dgl-reg --rm --hostname=reg-machine --runtime=nvidia -dit dgllib/dgl-ci-gpu:conda /bin/bash
+docker exec dgl-reg bash /root/asv_data/run.sh
+docker cp dgl-reg:/root/regression/dgl/asv/. /home/ubuntu/asv_data/  # Change /home/ubuntu/asv to the path you want to put the result
+docker stop dgl-reg
+```
+
+And in the directory you choose (such as `/home/ubuntu/asv_data`), there's a `html` directory. You can use `python -m http.server` to start a server to see the result