Make it less likely people will do the wrong thing

Make it very clear in every single place that the setting for ALE's own
completion implementation is mentioned that you should not enable it if
you want to use ALE as a completion source for other plugins like
Deoplete.

1 files changed, 3 insertions, 0 deletions

diff --git a/README.md b/README.md
index a35a62ab..1de2b412 100644
--- a/README.md
+++ b/README.md
@@ -174,6 +174,9 @@ other plugins, and can be enabled by changing a setting before ALE is loaded.
 ```vim
 " Enable completion where available.
 " This setting must be set before ALE is loaded.
+"
+" You should not turn this setting on if you wish to use ALE as a completion
+" source for other completion plugins, like Deoplete.
 let g:ale_completion_enabled = 1
 ```