Add information on how to contribute to the README

[Docs] Add information on how to contribute to the README

I noticed after reading a few GitHub comments that the information
about contributing to V8 is hard to find. I think it makes sense
to add this information to the README.

NOTRY=true
R=jochen@chromium.org, seththompson@chromium.org
LOG=N

Committed: https://crrev.com/8d77f788e7b626b9fda633e600405f720bfad8f7
Cr-Commit-Position: refs/heads/master@{#30730}

[Michael Hablich, 2015-09-14 18:15:39]

PTAL

[Michael Hablich, 2015-09-14 18:15:59]

hablich@chromium.org changed reviewers:
+ seththompson@chromium.org
- seththompsos@chromium.org

[jochen (gone - plz use gerrit), 2015-09-15 07:38:05]

lgtm

[Michael Hablich, 2015-09-15 07:51:38]

The CQ bit was checked by hablich@chromium.org

[commit-bot: I haz the power, 2015-09-15 07:51:50]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1342803004/1
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1342803004/1

[commit-bot: I haz the power, 2015-09-15 07:53:04]

Committed patchset #1 (id:1)

[commit-bot: I haz the power, 2015-09-15 07:53:20]

Patchset 1 (id:??) landed as
https://crrev.com/8d77f788e7b626b9fda633e600405f720bfad8f7
Cr-Commit-Position: refs/heads/master@{#30730}