[Docs][PrebidServer] Adding /version doc (#1198)

This CL introduces documentation for the `/version` admin endpoint.

Issue: #1197

prebid-server/endpoints/version.md

@@ -0,0 +1,29 @@
+## `GET /version`
+
+This endpoint exposes the application version as defined at compilation time.
+Version can be set either:
+- manually using go build -ldflags "-X main.Rev=`git rev-parse --short HEAD`"
+- automatically via .travis.yml configuration
+See https://github.com/prebid/prebid-server/blob/master/pbs_light.go:
+
+```go
+// Holds binary revision string
+// Set manually at build time using:
+//    go build -ldflags "-X main.Rev=`git rev-parse --short HEAD`"
+// Populated automatically at build / release time via .travis.yml
+//   `gox -os="linux" -arch="386" -output="{{.Dir}}_{{.OS}}_{{.Arch}}" -ldflags "-X main.Rev=`git rev-parse --short HEAD`" -verbose ./...;`
+// See issue #559
+var Rev string
+```
+
+### Sample responses
+
+#### Version set
+```json
+{"revision": "d6cd1e2bd19e03a81132a23b2025920577f84e37"},
+```
+
+#### Version not set
+```json
+{"revision": "not-set"}`
+```