Add jitpack to README

[aballano]

Adds jitpack as a gradle repository in order to fetch MnemoniK

[codecov]

Codecov Report

Merging #1083 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##             master   #1083      +/-   ##
===========================================
+ Coverage     42.38%   42.4%   +0.02%     
  Complexity      750     750              
===========================================
  Files           360     360              
  Lines          9989    9989              
  Branches       1082    1082              
===========================================
+ Hits           4234    4236       +2     
  Misses         5439    5439              
+ Partials        316     314       -2

Impacted Files	Coverage Δ	Complexity Δ
...tics/src/main/kotlin/arrow/optics/instances/map.kt	91.89% <0%> (+2.7%)	0% <0%> (ø)	⬇️
...ics/src/main/kotlin/arrow/optics/instances/mapk.kt	90.9% <0%> (+4.54%)	0% <0%> (ø)	⬇️

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