apt-add-repository: add page (#3229)

* apt-add-repository: add page

* Update apt-add-repository.md

pages/linux/apt-add-repository.md

@@ -0,0 +1,19 @@
+# apt-add-repository
+
+> Manages apt repository definitions.
+
+- Add a new apt repository:
+
+`apt-add-repository {{repository_spec}}`
+
+- Remove an apt repository:
+
+`apt-add-repository --remove {{repository_spec}}`
+
+- Update the package cache after adding a repository:
+
+`apt-add-repository --update {{repository_spec}}`
+
+- Enable source packages:
+
+`apt-add-repository --enable-source {{repository_spec}}`