Fix `unless_exists` example in README.md

[andrykonchin]

Close #657

Changes:

• fixed example in the README file
• documented unless_exists option of persisting methods
• refactored and removed if_exists option, that was available but is an implementation detail and isn't documented

[codecov[bot]]

Codecov Report

Merging #660 (8ca0e64) into master (c9574f4) will decrease coverage by 0.02%. The diff coverage is 100.00%.

:exclamation: Current head 8ca0e64 differs from pull request most recent head 464f115. Consider uploading reports for the commit 464f115 to get more accurate results

@@            Coverage Diff             @@
##           master     #660      +/-   ##
==========================================
- Coverage   90.34%   90.33%   -0.02%     
==========================================
  Files          62       62              
  Lines        3160     3156       -4     
==========================================
- Hits         2855     2851       -4     
  Misses        305      305              

Impacted Files	Coverage Δ
lib/dynamoid/adapter_plugin/aws_sdk_v3.rb	94.21% <ø> (-0.09%)	:arrow_down:
lib/dynamoid/config.rb	90.32% <100.00%> (ø)
lib/dynamoid/persistence.rb	98.17% <100.00%> (-0.02%)	:arrow_down:
lib/dynamoid/persistence/save.rb	96.36% <100.00%> (ø)
lib/dynamoid/persistence/update_fields.rb	100.00% <100.00%> (ø)

[github-actions[bot]]

Package	Line Rate	Health
dynamoid	90%	➖
Summary	90% (2851 / 3156)	➖

Minimum allowed line rate is 90%