README: Add a note on the J4 connector

diff --git a/README.md b/README.md
index 2b9fc67..2acf632 100644 (file)
--- a/README.md
+++ b/README.md
@@ -70,6 +70,10 @@ Pin    | Signal Name | Description
 Shield | DE9 Shield     | Connected to the SBC GND signal
 
 #### J4 - NVRAM Battery
+Connect 3V battery for SRAM backup to this connector.
+
+**Important: If the SRAM battery backup is not desired, short J4 with a jumper. Do not leave this connector open.**
+
 Pin | Signal Name | Description
 --- | ----------- | -----------
 1      | VBAT        | Positive terminal - +3V