add: changelog

[ryuash]

Description

closes #810

@kwunyeung @RiccardoM Can you guys confirm this calculates the apr correctly with the community tax taken in to consideration?

// total_supply * ((1 - community_tax) * inflation) / bonded
const inflationWithCommunityTax = Big(1).minus(communityTax).times(inflation).toPrecision(2); 
const apr = Big(rawSupplyAmount).times(inflationWithCommunityTax).div(bondedTokens).toNumber();

Checklist

• Ran Linting
• Targeted PR against correct branch.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Wrote unit tests.
• Added an entry to the CHANGELOG.md file.
• Re-reviewed Files changed in the Github PR explorer.