add a template for Phoenix projects

Co-Authored-By: JuanPablo <jpabloaj@gmail.com>
Co-Authored-By: Sean Callan <sean@seancallan.com>

community/Elixir/Phoenix.gitignore

+# gitignore template for Phoenix projects
+# website: http://www.phoenixframework.org/
+#
+# Recommended template: Elixir.gitignore
+
+# Temporary files
+/tmp
+
+# Static artifacts
+/node_modules
+/assets/node_modules
+
+# Since we are building assets from web/static,
+# we ignore priv/static. You may want to comment
+# this depending on your deployment strategy.
+/priv/static/
+
+# Installer-related files
+/installer/_build
+/installer/tmp
+/installer/doc
+/installer/deps