doc: Update install.md (#3077)
doc: Update install.md (#3077)

* doc: Update install.md

add `conda install Cython` before install the forked version of `pycocotools`

* add cython to requirements/build.txt

Co-authored-by: Kai Chen <cb0a7671c225c88fb1d102d95b0194ff7d8c6cde@gmail.com>
open-mmlab/mmdetection,open-mmlab/mmdetection