Added commit sign off section

This was in the main contributing guidelines, but is only enforced in this repo. I've added it here.

As part of ipfs/community#63

License: MIT
Signed-off-by: Richard Littauer <richard.littauer@gmail.com>

contribute.md

@@ -16,3 +16,77 @@ Please look and conform to our [Go Contribution Guidelines](https://github.com/i
 - Ask questions or talk about things in [Issues](https://github.com/ipfs/go-ipfs/issues) or #ipfs on freenode.
 - Ensure you are able to contribute (no legal issues please-- we'll probably setup a CLA).
 - Have fun!
+
+## Repository specific guidelines:
+
+### Commit messages
+
+Commit messages must start with a short subject line, followed by an optional, 
+more detailed explanatory text which is separated from the summary by an empty line. 
+We use [GitCop](https://gitcop.com) to check that commit messages are
+properly written. It checks the following:
+
+* The first line of a commit message, called the subject line should
+  not be more than 80 characters long.
+
+* The commit message should end with the following trailers:
+
+  ```
+  License: MIT
+  Signed-off-by: User Name <email@address>
+  ```
+
+  where "User Name" is the author's real (legal) name and
+  email@address is one of the author's valid email addresses.
+
+  These trailers mean that the author agrees with the 
+  [developer certificate of origin](docs/developer-certificate-of-origin)
+  and with licensing the work under the [MIT license](docs/LICENSE).
+
+  To help you automatically add these trailers, you can run the
+  [setup_commit_msg_hook.sh](dev/tools/hooks/setup_commit_msg_hook.sh)
+  script which will setup a Git commit-msg hook that will add the above
+  trailers to all the commit messages you write.
+
+See the [documentation about amending commits](docs/amending-commits.md)
+for explanation about how you can rework commit messages.
+
+Some example commit messages:
+
+```
+parse_test: improve tests with stdin enabled arg
+
+Now also check that we get the right arguments from
+the parsing.
+
+License: MIT
+Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
+```
+
+and
+
+```
+net/p2p + secio: parallelize crypto handshake
+
+We had a very nasty problem: handshakes were serial so incoming
+dials would wait for each other to finish handshaking. this was
+particularly problematic when handshakes hung-- nodes would not
+recover quickly. This led to gateways not bootstrapping peers
+fast enough.
+
+The approach taken here is to do what crypto/tls does:
+defer the handshake until Read/Write[1]. There are a number of
+reasons why this is _the right thing to do_:
+- it delays handshaking until it is known to be necessary (doing io)
+- it "accepts" before the handshake, getting the handshake out of the
+  critical path entirely.
+- it defers to the user's parallelization of conn handling. users
+  must implement this in some way already so use that, instead of
+  picking constants surely to be wrong (how many handshakes to run
+  in parallel?)
+
+[0] http://golang.org/src/crypto/tls/conn.go#L886
+
+License: MIT
+Signed-off-by: Juan Benet <juan@ipfs.io>
+```