Docs: Update settings file info with forum post

https://forum.sparkfun.com/viewtopic.php?f=116&t=59581#p241661

Author: nseidle

docs/configure_with_settings_file.md

@@ -10,6 +10,24 @@ All device settings are stored both in internal memory and an SD card if one is
 
 All system configuration can be done by editing the *SFE_[Platform]_Settings_0.txt* file (example shown above) where [Platform] is Facet, Express, Surveyor, etc and 0 is the profile number (0, 1, 2, 3). This file is created when a microSD card is installed. The settings are clear text but there are no safety guards against setting illegal states. It is not recommended to use this method unless You Know What You're Doing®.
 
+Keep in mind: 
+* The settings file contains hundreds of 300 settings.
+* The SD card file "SFE_Express_Settings_0.txt" is used for Profile 1, SD card file "SFE_Express_Settings_1.txt" is used for Profile 2, etc. (note that setting 0 is for profile 1, ...)
+* When switching to a new profile, the settings file on the SD card with all settings will be created or updated. The internal settings will not be updated until you switch to the profile. Additionally, the file for a particular profile will not be created on the SD card until you switch to that profile.
+* It is not necessary that the settings file on the SD card have all of the settings.
+
+For example, if you only wanted to set up two wireless networks for profile 2, you could create a file named "SFE_Express_Settings_1.txt" that only contained the following settings:
+
+    profileName=a name you choose
+    enableTcpServer=1
+    wifiNetwork0SSID=your SSID name 1
+    wifiNetwork0Password=your SSID password 1
+    wifiNetwork1SSID=your SSID name 2
+    wifiNetwork1Password=your SSID password 2
+    wifiConfigOverAP=0
+
+These settings on the SD card will overwrite the settings in the RTK Express internal memory. Once you select this profile on your RTK Express, the SD card file will be overwritten with all of the merged settings.
+
 ## Forcing a Factory Reset
 
 ![Setting size of settings to -1 to force reset](<img/SparkFun RTK Settings File - Factory Reset.png>)