Add project/0 warning (#14016)

Co-authored-by: Alex Naser <[email protected]>
Co-authored-by: José Valim <[email protected]>

lib/mix/lib/mix/project.ex

@@ -75,6 +75,14 @@ defmodule Mix.Project do
   the `:erlc_paths` configuration is used by `mix compile.erlang`, `mix compile.yecc`,
   and other tasks.
 
+  > #### Keep `project/0` fast {: .warning}
+  >
+  > `project/0` is called whenever your `mix.exs` is loaded, so heavy
+  > computation should be avoided. If a task requires a potentially complex
+  > configuration value, it should allow its configuration to be set to an
+  > anonymous function or similar, so that it can be invoked only when
+  > needed by the task itself.
+
   ## CLI configuration
 
   Mix is most often invoked from the command line. For this purpose, you may define