Security level settings are incorrectly documented in the yaml/readme files

[duncandrennan]

.yml and README.md have

0 - no auth required, 1 - encryption reqd, 2 - encryption + MITM reqd

Code has:

enum mgos_bt_gatt_sec_level {
  /* No authentication required */
  MGOS_BT_GATT_SEC_LEVEL_NONE = 0,
  /* Authenitcation required */
  MGOS_BT_GATT_SEC_LEVEL_AUTH = 1,
  /* Authentication and encryption required */
  MGOS_BT_GATT_SEC_LEVEL_ENCR = 2,
  /* Authentication, encryption and MITM protection required */
  MGOS_BT_GATT_SEC_LEVEL_ENCR_MITM = 3,
};

yaml & README need to be aligned. We picked this up when iOS threw errors while trying to use "1" with the assumption it was encrypted. With "2" iOS works as expected.