add dco section in contribution overview page (#266)

Author: dannypsnl

docs/contribute/overview.md

@@ -20,6 +20,39 @@ To help new contributors understand WasmEdge development workflow, this guide wi
 - [WasmEdge release process](release.md)
 - [Join WasmEdge community](community.md)
 
+## Contribution Flow
+
+To contribute to WasmEdge
+
+### Signing-off on Commits (Developer Certificate of Origin)
+
+To contribute to this project, you must agree to the Developer Certificate of Origin (DCO) for each commit you make. The DCO is a simple statement that you, as a contributor, have the legal right to make the contribution.
+
+See the [DCO](https://developercertificate.org) file for the full text of what you must agree to and how it works [here](https://github.com/probot/dco#how-it-works). To signify that you agree to the DCO for contributions, you simply add a line to each of your git commit messages:
+
+```text
+Signed-off-by: John Doe <[email protected]>
+```
+
+In most cases, you can add this signoff to your commit automatically with the `-s` or `--signoff` flag to `git commit`. You must use your real name and a reachable email address (sorry, no pseudonyms or anonymous contributions). An example of signing off on a commit:
+
+```bash
+git commit -s -m “my commit message w/signoff”
+```
+
+To ensure all your commits are signed, you may choose to add this alias to your global `.gitconfig`:
+
+```text
+[alias]
+  amend = commit -s --amend
+  cm = commit -s -m
+  commit = commit -s
+```
+
+Or you may configure your IDE, for example, Visual Studio Code to automatically sign-off commits for you:
+
+![VSCode sign-off](https://user-images.githubusercontent.com/7570704/64490167-98906400-d25a-11e9-8b8a-5f465b854d49.png)
+
 ## Ideas for contributions
 
 If you're looking for something to contribute, we have issues