Explain vendor vs proxy (gomods#1403)

* adding a section for discussion on when to do vendor directories vs. when to use athens or another proxy server

* adding more detail and an outline

* Adding a bit more 😄

* adding to vendor vs athens faq

* removing reference material from FAQ when to use Athens

* Fixing awkward phrasing in faq docs.

docs/content/faq.md

@@ -87,3 +87,23 @@ Which currently includes:
 - hg
 - bzr
 - fossil
+
+### When should I use a vendor directory, and when should I use Athens?
+
+The Go community has used vendor directories for a long time before module proxies like Athens came along, so naturally each group collaborating on code should decide for themselves whether they want to use a vendor directory, use Athens, or do both!
+
+Using a vendor directory (without a proxy) is valuable when:
+
+- CI/CD systems don't have access to an Athens (even if it's internal)
+- When the vendor directory is so small that it is still faster to check it out from a repo than it is to pull zip files from the server
+- If you're coming from glide/dep or another dependency management system that leveraged the vendor directory
+
+Athens (without a vendor directory) is valuable when:
+
+- You have a new project
+- Your team requires that you use Athens (i.e. for isolation or dependency auditing)
+- Your vendor directory is large and causing slow checkouts and downloading from Athens speeds the build up
+  - For developers slow checkouts will not be as much of a problem as for ci tools which frequently need to checkout fresh copies of the project
+- You want to remove the vendor directory from your project to:
+  - Reduce noise in pull requests
+  - Reduce difficulty doing fuzzy file searching in your project