docs(adr): ADR-009: Supply module (#786)

## Description

Closes: #XXXX

ADR-009 for supply module implementation.

---

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- [ ] included the correct [type prefix](https://github.com/commitizen/conventional-commit-types/blob/v3.0.0/index.json) in the PR title
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- [ ] added a changelog entry to `CHANGELOG.md`
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docs/architecture/adr-009-supply-module.md

@@ -0,0 +1,51 @@
+# ADR 009: Supply module
+
+## Changelog
+
+- March 21, 2022: Initial draft;
+- March 21, 2022: First review;
+- March 21, 2022: Second review.
+
+## Status
+
+DRAFTED
+
+## Abstract
+
+This ADR defines the `x/supply` module which will expose a set of APIs that can be called by data aggregator websites (such as CoinGecko and CoinMarketCap) in order to fetch updated data about a specific supply.
+
+## Context
+
+Currently, inside our [CoinGecko](https://www.coingecko.com/en/coins/desmos) and [CoinMarketCap](https://coinmarketcap.com/currencies/desmos/) some important information about current and total supply are missing or not correctly updated. To solve this, we can implement a series of APIs that read those data from the chain. Data aggregator websites that have the token listed can later use them.
+
+## Decision
+
+The APIs will be exposed in a new module called `x/supply` that will have the purpose to fetch the given information using different Cosmos SDK modules (namely `x/bank`, `x/distribution`, `x/staking`) and apply some conversions on them in order to display them the best way possible for the client.
+
+### Queries
+
+All the following APIs will have a custom param named `multiplier` that allows to set the multiplier to be used when returning the values. A `multiplier` of `0` will identify the whole token amount, while a multiplier of `1.000.000` will make the result display the values in millionth of units.
+
+#### Total Supply
+
+This query will fetch the total supply of a given token `denom`.
+
+#### Current supply
+
+This query will return the circulating supply by subtracting the total vested tokens amount and community pool amount from the total supply of the given token.
+
+## Consequences
+
+### Positive
+- All the chain nodes will expose these APIs automatically after their node updates;
+- No state-breaking changes introduced;
+- Easily extensible.
+
+## Test Cases
+
+- Check API to correctly return the converted total Supply from millionth to non-millionth representation;
+- Check API to correctly return the current supply.
+
+## References
+
+- Issue [#733](https://github.com/desmos-labs/desmos/issues/773).