Add documentation for adding GC through API

[Roshyo]

Fixes #232

[codecov[bot]]

Codecov Report

Merging #233 (4eff5e7) into 0.12.x (8a80848) will increase coverage by 0.11%. The diff coverage is n/a.

@@             Coverage Diff              @@
##             0.12.x     #233      +/-   ##
============================================
+ Coverage     49.53%   49.65%   +0.11%     
  Complexity      534      534              
============================================
  Files           103      103              
  Lines          1716     1724       +8     
============================================
+ Hits            850      856       +6     
- Misses          866      868       +2     

Impacted Files	Coverage Δ
src/DependencyInjection/Configuration.php	100.00% <0.00%> (ø)
src/Form/Type/GiftCardConfigurationType.php	0.00% <0.00%> (ø)
...endencyInjection/SetonoSyliusGiftCardExtension.php	85.71% <0.00%> (+1.50%)	:arrow_up:

---

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 8a80848...4eff5e7. Read the comment docs.

[loevgaard]

You can merge this despite failing pipes (because it's only *.md files)