Added Terminus documentation

docs/terminus.md

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+# Terminus: Decentralized authorization
+
+<img src="https://s3.amazonaws.com/static.simiotics.com/dao/terminus-ufo.png" width="250"/>
+
+Terminus is a smart contract which any application can use to track user permissions on Ethereum-based blockchains.
+
+Applications create pools of tokens which represents levels of access to their functionality.
+When a blockchain address attempts to use any particular functionality of such an application, the application
+checks the token balance of that address to understand whether or not the address is authorized to
+access the desired functionality.
+
+Applications can elect to make the tokens in their authorization pools tradable. This allows their users
+to trade their access on free, decentralized markets like Open Sea.
+
+Applications can also choose whether or not tokens from their authorization pools get burned when they are
+used. This allows them to create authorization pools for limited-use resources (e.g. NFT whitelists)
+or for renewable resources (e.g. badges in blockchain games).
+
+The Terminus interface is an extension of the [ERC1155 multi-token interface](https://eips.ethereum.org/EIPS/eip-1155).
+
+## Benefits
+
+### Open markets for authorization
+
+Because Terminus implements the `ERC1155` interface, Terminus authorization tokens can be traded on open
+markets.
+
+This is good for users because purchasing access to an application is no longer a sunk cost. It introduces
+a sense of liquidity to the notion of access.
+
+For authorizing applications which elect to make their tokens tradable:
+1. The market for authorization tokens is the best possible price discovery mechanism for their product
+or service.
+2. They can forecast usage of their application based on the activity of their authorization tokens on
+the market.
+3. They operate much closer to capacity because users who are no longer engaged with the application can now
+make their authorization available to those with intent to use it.
+
+#### Example: NFT whitelists
+
+Even on popular NFT projects, fewer than 50% of the whitelisted addresses typically exercise their whitelist
+privileges. Getting on an NFT whitelist generally requires some effort on the part of a user -- it is
+something they have to compete for. If they then do not exercise those privileges -- because they lack
+the means or the interest -- it represents a total waste of that effort.
+
+If an NFT project used Terminus to manage its whitelist, these users could sell their whitelist access
+on Open Sea, making their effort worthwhile. This would benefit the NFT project, too, by guaranteeing
+engaged demand for their tokens.
+
+### Operationally simple
+
+Managing authorizations on a smart contract is not easy, especially if those authorizations heavily
+involve state.
+
+The only purpose of Terminus is to manage authorizations and authorization state. This means that, over time,
+it will achieve maximal security and minimal gas overhead for this task.
+
+Terminus exposes its functionality through the simple ERC1155 interface, which makes it very easy to integrate with.
+
+Terminus comes with a web interface, which makes it easy for a team to inspect and manage their authorization
+pools collaboratively.
+
+### Transparent
+
+Checking whether an address is authorized to perform an action only requires an `ERC1155` `balanceOf`
+call against the relevant Terminus authorization pool.
+
+This means that it is transparent to both the application and the user whether or not they have permission
+to do something on the application.
+
+Users can inspect the configuration of each pool to understand the authorization semantics - for example,
+if their authorization tokens are burned on use or if they persist.
+
+## The Terminus smart contract
+
+Our reference implementation of Terminus is also the one we are using in production. It is implemented
+as a facet on an [EIP2535 proxy](https://eips.ethereum.org/EIPS/eip-2535).
+
+The entrypoint to understanding Terminus functionality is the [`TerminusFacet`](../contracts/terminus/TerminusFacet.sol).
+
+## Status and roadmap
+
+Terminus is currently live on the [Polygon Mumbai testnet](../operations/README.md) and is being used by
+our earliest adopters.
+
+We are currently building our roadmap. Our immediate priorities can be found in this repository's issues
+using the `terminus` tag: https://github.com/bugout-dev/dao/issues?q=is%3Aissue+is%3Aopen+label%3Aterminus