[Enhance] Add numba and numpy version requirements in FAQ (#379)

* add numba and numpy version problem to FAQ

* delete duplicate docs

* modify numpy->numba

* add workaround

* numpy version < 1.20.0

* modify docstring

* Update faq.md
Co-authored-by: Wenwei Zhang <40779233+ZwwWayne@users.noreply.github.com>

docs/faq.md

@@ -16,3 +16,9 @@ We list some potential troubles encountered by users and developers, along with
   pip uninstall pycocotools mmpycocotools
   pip install mmpycocotools
   ```
+
+- If you face the error shown below, and your environment contains numba == 0.48.0 with numpy >= 1.20.0
+
+  ``TypeError: expected dtype object, got 'numpy.dtype[bool_]'``
+
+  please downgrade numpy to < 1.20.0 or install numba == 0.48 from source, because in numpy==1.20.0, `np.dtype` produces subclass due to API change.