Add zenodo DOI badge

[mraspaud]

Adds the DOI badge in the readme

• Closes Citing satpy #829

[codecov]

Codecov Report

Merging #833 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #833      +/-   ##
=========================================
- Coverage    83.9%   83.9%   -0.01%     
=========================================
  Files         167     167              
  Lines       24445   24445              
=========================================
- Hits        20511   20510       -1     
- Misses       3934    3935       +1

Impacted Files	Coverage Δ
satpy/readers/avhrr_l1b_gaclac.py	90.14% <0%> (-1.41%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 2654cf3...7bd07b1. Read the comment docs.

[coveralls]

Coverage increased (+0.01%) to 83.899% when pulling 9f8bf67 on mraspaud:add-doi-badge into 2654cf3 on pytroll:master.

[coveralls]

Coverage increased (+0.02%) to 83.903% when pulling 7bd07b1 on mraspaud:add-doi-badge into 2654cf3 on pytroll:master.