add min_rf feature in add method

[alihossein]

I add a new feature called min_rf (Doc) in Pysolr because I need it.

I hope this was useful.

[codecov]

Codecov Report

Merging #350 into master will decrease coverage by 0.06%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #350      +/-   ##
==========================================
- Coverage   94.31%   94.25%   -0.07%     
==========================================
  Files           7        7              
  Lines        1338     1340       +2     
==========================================
+ Hits         1262     1263       +1     
- Misses         76       77       +1     

Impacted Files	Coverage Δ
pysolr.py	89.33% <50.00%> (-0.12%)	⬇️

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