docs: how to avoid jagged edges with mask painting

[unitycoder]

add wiki page for the instructions

*started here: https://github.com/unitycoder/UnityMobilePaint/wiki/How-to-avoid-white---jagged-edges%3F

[unitycoder]

issue

[unitycoder]

one solution: enable [x] Use Threshold and set Paint Threshold to 250-254

[unitycoder]

dont use jpegs, compression artifacts:

[unitycoder]

if using bilinear filtering on texture import settings, can cause issues:

if using point filter instead: