Add duecredit citation

[rly]

Motivation

Fix #476.

How to test the behavior?

Create python script that imports hdmf. Then:

pip install duecredit
python -m duecredit yourscript.py
duecredit summary

Output:

DueCredit Report:
- HDMF: Hierarchical Data Modeling Framework for Modern Science Data Standards / hdmf/ (v 2.2.0.post.dev81) [1]
- Scientific tools library / numpy (v 1.19.1) [2]

2 packages cited
0 modules cited
0 functions cited

References
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[1] Tritt, A.J. et al., 2019. HDMF: Hierarchical Data Modeling Framework for Modern Science Data Standards. In 2019 IEEE International Conference on Big Data (Big Data). pp. 165–179.
[2] Van Der Walt, S., Colbert, S.C. & Varoquaux, G., 2011. The NumPy array: a structure for efficient numerical computation. Computing in Science & Engineering, 13(2), pp.22–30.

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