add README

Author: champo

README.md

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+![muun](https://muun.com/images/github-banner-v2.png)
+
+## About
+
+This is the source code repository for muun's wallet core library. Muun is a non-custodial 2-of-2 multisig wallet with a special focus on security and ease of use.
+
+This library is used by our mobile wallets with [gomobile](https://godoc.org/golang.org/x/mobile/cmd/gomobile).
+
+## Responsible Disclosure
+
+Send us an email to report any security related bugs or vulnerabilities at [[email protected]](mailto:[email protected]).
+
+You can encrypt your email message using our public PGP key.
+
+Public key fingerprint: `1299 28C1 E79F E011 6DA4 C80F 8DB7 FD0F 61E6 ED76`
+