Sync .gitignore with other systems

.gitignore

@@ -1,9 +1,22 @@
-/_build
-/cover
-/deps
+# The directory Mix will write compiled artifacts to.
+/_build/
+
+# If you run "mix test --cover", coverage assets end up here.
+/cover/
+
+# The directory Mix downloads your dependencies sources to.
+/deps/
+
+# Where 3rd-party dependencies like ExDoc output generated docs.
+/doc/
+
+# Ignore .fetch files in case you like to edit your project deps locally.
+/.fetch
+
+# If the VM crashes, it generates a dump, let's ignore it too.
 erl_crash.dump
-*.ez
-build.log
-archive.log
-.nerves
-/doc
+
+# Ignore our generated files
+/build.log
+/archive.log
+/.nerves