Support MongoDB with djongo backend

[Nagico]

feat:

• MongoDB: using mongdb:// and mongodb+srv:// schemes
• readme.rst: add usage about MongoDB

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More information

• djongo document
• pymongo document
• mongodb uri format

[codecov]

Codecov Report

Merging #204 (0ae2f73) into master (698882e) will decrease coverage by 0.24%.
The diff coverage is 92.00%.

❗ Current head 0ae2f73 differs from pull request most recent head bc8da3d. Consider uploading reports for the commit bc8da3d to get more accurate results

@@            Coverage Diff             @@
##           master     #204      +/-   ##
==========================================
- Coverage   92.13%   91.89%   -0.25%     
==========================================
  Files           1        1              
  Lines          89      111      +22     
  Branches       17       24       +7     
==========================================
+ Hits           82      102      +20     
- Misses          3        4       +1     
- Partials        4        5       +1     

Impacted Files	Coverage Δ
dj_database_url.py	91.89% <92.00%> (-0.25%)	⬇️

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