Add Code of Conduct (#17)

* Add link to COC

**what is the change?:**
Adding a document linking to the Facebook Open Source Code of Conduct.

**why make this change?:**
Facebook Open Source provides a Code of Conduct statement for all
projects to follow.

**test plan:**
Viewing it on my branch.

**issue:**
internal task t23481323

* Add CODE_OF_CONDUCT.md

**what is the change?:**
Adding a document linking to the Facebook Open Source Code of Conduct,
for visibility and to meet Github community standards.

**why make this change?:**
Exposing the COC via a separate markdown file is a standard being
promoted by Github via the Community Profile in order to meet their Open
Source Guide's recommended community standards.

As you can see, adding this file will improve [facebook-nodejs-ads-sdk's Community Profile](https://github.com/facebook/facebook-nodejs-ads-sdk/community)
checklist and increase the visibility of our COC.

**test plan:**
Viewing it on my branch -
(Flarnie will insert screenshot)

**issue:**
internal task t23481323

* Reduce repetition in `CONTRIBUTING.md`

Rather than repeating the exact same wording in the `CONTRIBUTING.md` as in `CODE_OF_CONDUCT.md` we can link one to the other. As per @JoelMarcey's suggestion.

* improve link url for COC

Author: flarnie

CODE_OF_CONDUCT.md

@@ -0,0 +1,3 @@
+# Code of Conduct
+
+Facebook has adopted a Code of Conduct that we expect project participants to adhere to. Please [read the full text](https://code.facebook.com/codeofconduct) so that you can understand what actions will and will not be tolerated.

CONTRIBUTING.md

@@ -2,6 +2,10 @@
 We want to make contributing to this project as easy and transparent as
 possible.
 
+## Code of Conduct
+
+The code of conduct is described in [`CODE_OF_CONDUCT.md`](CODE_OF_CONDUCT.md)
+
 ## Our Development Process
 The Facebook Ads API SDK for NodeJS is currently developed in Facebook's internal
 repositories and then exported out to GitHub by a Facebook team member. We invite