add to docs: troubleshooting phone connectivity to POSM

[danbjoseph]

Phone is connected to the POSM wifi but ODK Collect errors when trying to fetch/send forms (see posm/posm#349).

• did you dismiss a "sign in to network" notification? is there an x on the wifi icon?
  • go to wifi wireless settings and click to login to the network
  • on the login page in the previous step go to the menu and select "Use this network as is"
• phone still won't connect
  • try toggling airplane mode on the phone