Added responsible discloure policy

Summary: Feedback from Bitcoin-ABC/bitcoinabc.org#24 indicated the disclosure policy should be part of the repo itself.  I totally agree.

Test Plan:
1. Read it carefully.
2. Try it out yourself.

Reviewers: deadalnix, schancel, Fabien, Mengerian, #bitcoin_abc

Reviewed By: deadalnix, Fabien, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1993

CONTRIBUTING.md

@@ -194,3 +194,8 @@ By contributing to this repository, you agree to license your work under the
 MIT license unless specified otherwise in `contrib/debian/copyright` or at
 the top of the file itself. Any work contributed where you are not the original
 author must contain its license header with the original author(s) and source.
+
+Disclosure Policy
+-----------------
+
+See [DISCLOSURE_POLICY](DISCLOSURE_POLICY).

DISCLOSURE_POLICY.md

@@ -0,0 +1,78 @@
+# Responsible Disclosure Policy
+
+Bitcoin ABC takes security very seriously.  We greatly appreciate any and all disclosures of bugs and vulnerabilities that are done in a responsible manner.  We will engage responsible disclosures according to this policy and put forth our best effort to fix disclosed vulnerabilities as well as reaching out to numerous node operators to deploy fixes in a timely manner.
+
+## Responsible Disclosure Guidelines
+
+Do not disclose any bug or vulnerability on public forums, message boards, mailing lists, etc. prior to responsibly disclosing to Bitcoin ABC and giving sufficient time for the issue to be fixed and deployed.
+Do not execute on or exploit any vulnerability.  This includes testnet, as both mainnet and testnet exploits are effectively public disclosure.  Regtest mode may be used to test bugs locally.
+
+## Reporting a Bug or Vulnerability
+
+When reporting a bug or vulnerability, please provide the following to security@bitcoinabc.org:
+A short summary of the potential impact of the issue (if known).
+Details explaining how to reproduce the issue or how an exploit may be formed.
+Your name (optional).  If provided, we will provide credit for disclosure.  Otherwise, you will be treated anonymously and your privacy will be respected.
+Your email or other means of contacting you.
+A PGP key/fingerprint for us to provided encrypted responses to your disclosure.  If this is not provided, we cannot guarantee that you will receive a response prior to a fix being made and deployed.
+
+## Encrypting the Disclosure
+
+We highly encourage all disclosures to be encrypted to prevent interception and exploitation by third-parties prior to a fix being developed and deployed.  Please encrypt using this PGP public key with id: `3199472CFB54790D90B8ECB4B1606D6B42B7F4C7`
+
+It may be obtained via:
+```
+gpg --recv-keys 3199472CFB54790D90B8ECB4B1606D6B42B7F4C7
+```
+
+Below are some basic instructions for encrypting your disclosure on Linux if you are unfamiliar with GPG:
+
+1. If you don’t already have a PGP key, first download GPG:
+For Debian based distributions:
+```
+sudo apt-get install gpg
+```
+For Archlinux based distributions:
+```
+pacman -S gnupg
+```
+2. Generate a PGP key:
+```
+gpg --full-generate-key
+```
+3. Select “RSA and RSA”
+4. Enter a key size of 4096.
+5. Follow the remaining prompts.
+6. Save your disclosure report to a plain text file, then encrypt:
+```
+gpg --output mydisclosurefile.asc --encrypt --recipient security@bitcoinabc.org mydisclosurefile
+```
+
+## Backup PGP Keys
+
+These PGP fingerprints and emails are provided only as backups in case you are unable to contact Bitcoin ABC via the security email above.
+
+#### Amaury Sechet
+```
+Bitcoin ABC Lead Developer
+deadalnix at gmail dot com
+629D7E5DDDA0512BD5860F2C5D7922BBD649C4A7
+```
+
+#### Shammah Chancellor
+```
+Bitcoin ABC Developer
+disclosure at shablag dot com
+7A55A44F3A3239827C8A594E7D3958C44427674A
+```
+
+#### Jason B. Cox
+```
+Bitcoin ABC Developer
+contact at jasonbcox dot com
+3BB16D00D9A6D281591BDC76E4486356E7A81D2C
+```
+
+## Making changes to this disclosure
+
+Note that any changes to this disclosure should be mirrored in a pull request to the [bitcoinabc.org repo](https://github.com/Bitcoin-ABC/bitcoinabc.org).

README.md

@@ -41,4 +41,9 @@ This Github repository contains only source code of releases.
 
 Bitcoin ABC development takes place at https://reviews.bitcoinabc.org/
 
-If you would like to contribute, please read [CONTRIBUTING](CONTRIBUTING.md)
+If you would like to contribute, please read [CONTRIBUTING](CONTRIBUTING.md).
+
+Disclosure Policy
+-----------------
+
+See [DISCLOSURE_POLICY](DISCLOSURE_POLICY).