remove applications: advice

It should be auto-inferred, and if not, extra_applications is correct instead: https://elixirforum.com/t/why-do-we-use-application-function-in-mix-exs/23834/2

Author: mweidner037

README.md

@@ -73,14 +73,6 @@ def deps do
 end
 ```
 
-You should also update your applications to include both projects:
-
-```elixir
-def application do
-  [applications: [:logger, :mongodb_ecto, :ecto]]
-end
-```
-
 To use the adapter in your repo:
 
 ```elixir