add devDependencies documentation

Author: kostiakoval

Documentation/Package.swift.md

@@ -54,6 +54,22 @@ let package = Package(
 )
 ```
 
+A package can require dependencies that are only needed during develop,
+as example for testing purposes. `devDependencies` are only fetched 
+when you build current package. They are not fetched if a package is 
+specified as a dependency in other package.
+
+```swift
+import PackageDescription
+
+let package = Package(
+ name: "Hello",
+ devDependencies: [
+ .Package(url: "ssh://git@example.com/Tester.git", versions: Version(1,0,0)..<Version(2,0,0)),
+ ]
+)
+```
+
 ## Customizing Builds
 
 That the manifest is Swift allows for powerful customization, for example: