Merge PR #1588: Add SECURITY.MD

SECURITY.md

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+# Security
+
+As part of our [Coordinated Vulnerability Disclosure
+Policy](https://tendermint.com/security), we operate a bug bounty.
+See the policy for more details on submissions and rewards.
+
+The following is a list of examples of the kinds of bugs we're most interested in for
+the Cosmos SDK. See [here](https://github.com/tendermint/tendermint/blob/master/SECURITY.md) for vulnerabilities we are interested in for Tendermint, and lower-level libraries, e.g. IAVL.
+
+## Modules 
+- x/staking
+- x/slashing
+- x/types
+- x/gov
+
+We are interested in bugs in other modules, however the above are most likely to have 
+significant vulnerabilities, due to the complexity / nuance involved
+
+## How we process Tx parameters
+- Integer operations on tx parameters, especially sdk.Int / sdk.Uint
+- Gas calculation & parameter choices 
+- Tx signature verification (code in x/auth/ante.go)
+- Possible Node DoS vectors. (Perhaps due to Gas weighting / non constant timing)
+
+## Handling private keys
+- HD key derivation, local and Ledger, and all key-management functionality
+- Side-channel attack vectors with our implementations
+  - e.g. key exfiltration based on time or memory-access patterns when decrypting privkey
+