docs: update install instruction

Was missing the assets:precompile part (cf. #190 (comment))

Without it I get 500 error on the app with very few informative logs...

README.md

@@ -445,6 +445,7 @@ And then execute:
 bundle
 bin/rails decidim:upgrade
 bin/rails db:migrate
+bin/rails assets:precompile
 ```
 
 Go to `yourdomain/admin/decidim_awesome` and start tweaking things!