feat(sdk): update README and release 1.0.0

Updates the SDK README and includes a changeset that will trigger the
1.0.0 release of the SDK.

.changeset/odd-grapes-wonder.md

+---
+'@eth-optimism/sdk': major
+---
+
+Update README and bump SDK to 1.0.0

packages/sdk/README.md

@@ -4,19 +4,33 @@
 
 The `@eth-optimism/sdk` package provides a set of tools for interacting with Optimism.
 
-## NOTICE
-
-WARNING: This package is currently under construction.
-You can definitely try to use it, but you probably won't have a good time.
-Lots of pieces are missing.
-We will announce the 1.0.0 release on Discord and on Twitter once it's ready, keep an eye out!
-
 ## Installation
 
 ```
 npm install @eth-optimism/sdk
 ```
 
-## API
+## Docs
+
+You can find auto-generated API documentation over at [sdk.optimism.io](https://sdk.optimism.io).
+
+## Using the SDK
+
+### CrossChainMessenger
+
+The [`CrossChainMessenger`](https://github.com/ethereum-optimism/optimism/blob/develop/packages/sdk/src/cross-chain-messenger.ts) class simplifies the process of moving assets and data between Ethereum and Optimism.
+You can use this class to, for example, initiate a withdrawal of ERC20 tokens from Optimism back to Ethereum, accurately track when the withdrawal is ready to be finalized on Ethereum, and execute the finalization transaction after the challenge period has elapsed.
+The `CrossChainMessenger` can handle deposits and withdrawals of ETH and any ERC20-compatible token.
+Detailed API descriptions can be found at [sdk.optimism.io](https://sdk.optimism.io/classes/crosschainmessenger).
+The `CrossChainMessenger` automatically connects to all relevant contracts so complex configuration is not necessary.
+
+### L2Provider and related utilities
+
+The Optimism SDK includes [various utilities](https://github.com/ethereum-optimism/optimism/blob/develop/packages/sdk/src/l2-provider.ts) for handling Optimism's [transaction fee model](https://community.optimism.io/docs/developers/build/transaction-fees/).
+For instance, [`estimateTotalGasCost`](https://sdk.optimism.io/modules.html#estimateTotalGasCost) will estimate the total cost (in wei) to send at transaction on Optimism including both the L2 execution cost and the L1 data cost.
+You can also use the [`asL2Provider`](https://sdk.optimism.io/modules.html#asL2Provider) function to wrap an ethers Provider object into an `L2Provider` which will have all of these helper functions attached.
+
+### Other utilities
 
-TODO: Fill this in once the initial package version is finished.
+The SDK contains other useful helper functions and constants.
+For a complete list, refer to the auto-generated [SDK documentation](https://sdk.optimism.io/)