Update readme.md - different env in ios (#281)

The use of /tmp/envfile is a bad idea, especially when there is one machine doing multiple builds on the same machine in parallel (eg. build server). 

IMHO it is much better to copy the correct .env.staging file to the default .env file. In this way we are not modifying system-wide files like /tmp/envfile.

Do you guys agree?

Co-authored-by: luancurti <luancurti@gmail.com>

README.md

@@ -253,11 +253,9 @@ Then edit the newly created scheme to make it use a different env file. From the
 - Click "Pre-actions", and under the plus sign select "New Run Script Action"
 - Where it says "Type a script or drag a script file", type:
   ```
-  echo ".env.staging" > /tmp/envfile   # replace .env.staging for your file
+  cp ${PROJECT_DIR}/../.env.staging .env  # replace .env.staging for your file
   ```
 
-This is still a bit experimental and dirty – let us know if you have a better idea on how to make iOS use different configurations opening a pull request or issue!
-
 ## Troubleshooting
 
 ### Problems with Proguard