Add documentation about prune to Gopkg.toml docs

docs/FAQ.md

@@ -347,7 +347,7 @@ It's up to you:
 **Cons**
 
 - Your repo will be bigger, potentially a lot bigger,
-  though `dep prune` can help minimize this problem.
+  though [`prune`](Gopkg.toml.md#prune) can help minimize this problem.
 - PR diffs will include changes for files under `vendor/` when Gopkg.lock is modified,
   however files in `vendor/` are [hidden by default](https://github.com/github/linguist/blob/v5.2.0/lib/linguist/generated.rb#L328) on Github.
 

docs/Gopkg.toml.md

@@ -58,6 +58,35 @@ system1-data = "value that is used by a system"
 system2-data = "value that is used by another system"
 ```
 
+## `prune`
+`prune` defines the global and per-project prune options for dependencies. The options control which files are not kept when writing the `vendor/` tree.
+
+The following is the current available options:
+* `unused-packages` prunes files in unused packages.
+* `non-go` prunes files that are not used by Go.
+* `go-tests` prunes Go test files.
+
+Some files are preversed by default (check the [isPreservedFile](../gps/prune.go#L254) function for the details).
+
+Prune options are off by default and can be turned on by setting them to `true` at the root level.
+```toml
+[prune]
+  non-go = true
+```
+
+The same prune options can be defined per-project. An addtional `name` field is required and should represent a project and not a package.
+
+
+```toml
+[prune]
+  non-go = true
+
+  [[prune.project]]
+    name = "github.com/project/name"
+    go-tests = true
+    non-go = false
+```
+
 ## `constraint`
 A `constraint` provides rules for how a [direct dependency](FAQ.md#what-is-a-direct-or-transitive-dependency) may be incorporated into the
 dependency graph.
@@ -171,4 +200,11 @@ codename = "foo"
 
   [metadata]
   propertyX = "valueX"
+
+[prune]
+  unused-packages = true
+
+  [[prune.project]]
+    name = "github.com/user/project2"
+    unused-packages = false
 ```