RAD-1766: Docs for adding a bidder adapter to Prebid Server (prebid#324)

* Start 'Add a Bidder Adapter to Prebid Server'

* Finish first draft of Prebid Server adapter docs

* Add final (mandatory) integration step

dev-docs/add-a-prebid-server-adapter.md

@@ -0,0 +1,96 @@
+---
+layout: page
+title: Add a Bidder Adapter to Prebid Server
+description: Add a Bidder Adapter to Prebid Server
+pid: 28
+top_nav_section: dev_docs
+nav_section: prebid-server
+---
+
+<div class="bs-docs-section" markdown="1">
+
+# Add a Bidder Adapter to Prebid Server
+{:.no_toc}
+
+This page describes how to write the code for a bidder adapter that integrates with [Prebid Server]({{site.baseurl}}/dev-docs/get-started-with-prebid-server.html).
+
+* TOC
+{:toc}
+
+## Step 1. Get the source code and build the API docs
+
+When writing your adapter it may be useful to refer to:
+
+- Autogenerated API documentation
+- Existing bidder adapter code
+
+For autogenerated API documentation using [`godoc`](http://godoc.org/golang.org/x/tools/cmd/godoc), run the following commands:
+
+```
+$ git clone https://github.com/prebid/prebid-server
+$ cd prebid-server/
+$ godoc ./adapters/
+```
+
+To read the source code of the existing adapters, see [the prebid-server repo](https://github.com/prebid/prebid-server/tree/master/adapters) (either on GitHub or your own machine, of course).
+
+## Step 2. Build Prebid Server and run the tests
+
+To build Prebid Server, see the instructions in the [README](https://github.com/prebid/prebid-server/tree/master/README.md).
+
+As of this writing, when you run `go test`, you end up testing everything in the project. To test just the adapter code, run:
+
+```
+$ go test ./adapters/
+```
+
+If everything goes as planned, you should see something like:
+
+```
+ok  	github.com/prebid/prebid-server/adapters	0.098s
+```
+
+A failing test will look something like the following (I triggered this failure by changing the bidder code for the AppNexus adapter to an invalid value):
+
+```
+ --- FAIL: TestAppNexusBasicResponse (0.01s)
+	appnexus_test.go:326: ParsePBSRequest returned invalid bidder
+body {"account_id":"","tid":"","cache_markup":0,"sort_bids":0,"max_key_length":0,"secure":0,"timeout_millis":0,"ad_units":[{"code":"first-tag","is_top_frame":0,"sizes":[{"w":300,"h":250}],"bids":[{"bidder":"audienceNetwork","bid_id":"random-id-from-pbjs-0","params":{"placementId":"987654321_999998888"}}],"config_id":""},{"code":"second-tag","is_top_frame":0,"sizes":[{"w":300,"h":250}],"bids":[{"bidder":"audienceNetwork","bid_id":"random-id-from-pbjs-1","params":{"placementId":"987654321_66775544"}}],"config_id":""}],"is_debug":false,"app":null,"device":null,"Start":"0001-01-01T00:00:00Z"}
+```
+
+## Step 3. Understand the adapter interface
+
+Now that you have the `godoc` output (and have [`adapter.go`](https://github.com/prebid/prebid-server/tree/master/adapters/adapter.go) available to read), you can see that the interface you need to implement is:
+
+{: .table .table-bordered .table-striped }
+| Name              | Description                                                                                                                                                                       |
+|-------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
+| `Name`            | Returns the code name of the adapter, a string -- must match the bidder code in Prebid.js                                                                                         |
+| `FamilyName`      | Returns a string used to build the OpenRTB bid request; used for cookie management                                                                                                |
+| `GetUsersyncInfo` | Returns user sync info for this bidder.  For more information about user syncing, see [What is Cookie Syncing and How Does it Work?](http://clearcode.cc/2015/12/cookie-syncing/) |
+| `SkipNoCookies`   | Returns a boolean value that answers: Should this adapter be called if there is no user ID on the impression?                                                                     |
+| `Call`            | Takes the request from the client and produces a bid request that is then sent to the adapter's external server.                                                                  |
+
+Other things to keep in mind include:
+
+- Your adapter must be in the `adapters` package
+- Your adapter's constructor (e.g., `NewYourBidderAdapter`) needs to accept an HTTP adapter config that controls how it makes requests to your bidder
+
+## Step 4. Write code and tests
+
+There's enough variation among bidders that we can't tell you exactly how to write the code.  To see how it's done, refer to the [adapters in the repo](https://github.com/prebid/prebid-server/tree/master/adapters/), their tests, and the interface documentation.
+
+The file naming conventions are:
+
++ `yourbidder.go`: implementation
++ `yourbidder_test.go`: tests
+
+## Step 5. Register your adapter with Prebid Server
+
+Finally, in order for Prebid Server to integrate your adapter, it has to be added to the list of adapters in [the implementation of `setupExchanges`](https://github.com/prebid/prebid-server/blob/master/pbs_light.go#L625).
+
+## Further Reading
+
++ [Get Started with Prebid Server]({{site.baseurl}}/dev-docs/get-started-with-prebid-server.html)
+
+</div>