Update dependency-specification.md

Add description of `source` property.

Co-authored-by: Arun Babu Neelicattu <arun.neelicattu@gmail.com>
Co-authored-by: Gabriel Galli <ggabriel96@hotmail.com>

docs/dependency-specification.md

@@ -152,6 +152,30 @@ with the corresponding `add` call:
 poetry add https://example.com/my-package-0.1.0.tar.gz
 ```
 
+## `source` dependencies
+
+To depend on a package from an [alternate repository](/docs/repositories/#install-dependencies-from-a-private-repository),
+you can use the `source` property:
+
+```toml
+[[tool.poetry.source]]
+name = "foo"
+url = "https://foo.bar/simple/"
+secondary = true
+
+[tool.poetry.dependencies]
+my-cool-package = { version = "*", source = "foo" }
+```
+
+with the corresponding `add` call:
+
+```sh
+poetry add my-cool-package --source foo
+```
+
+!!!note
+
+    In this example, we expect `foo` to be configured correctly. See [using a private repository](repositories.md#using-a-private-repository) for further information.
 
 ## Python restricted dependencies
 
@@ -178,7 +202,6 @@ via the `markers` property:
 pathlib2 = { version = "^2.2", markers = "python_version ~= '2.7' or sys_platform == 'win32'" }
 ```
 
-
 ## Multiple constraints dependencies
 
 Sometimes, one of your dependency may have different version ranges depending