updated the usage.

README.md

@@ -34,18 +34,22 @@ the --downlink option should be specified.
 
 ## Usage
 
-    lorawan_toa.py [-h] [--band-width NUMBER] [--disable-auto-ldro]
-                   [--enable-ldro] [--disable-eh] [--downlink]
-                   [--disable-crc] [--cr NUMBER] [--preamble NUMBER]
-                   [--duty-cycle NUMBER] [-v] [-d]
+```
+usage: toa.py [-h] [--band-width NUMBER] [--disable-auto-ldro] [--enable-ldro]
+              [--disable-eh] [--downlink] [--disable-crc] [--cr NUMBER]
+              [--preamble NUMBER] [--duty-cycle NUMBER] [-v] [-d]
               SF SIZE
 
 LoRa Time on Air calculator.
 
 positional arguments:
   SF                   Spreading Factor. It should be from 7 to 12.
-      SIZE                 PHY payload size in byte. It's equal to the MAC payload
-                           + 5.
+  SIZE                 PHY payload size in byte. Remember that PHY payload
+                       (i.e. MAC frame) is consist of MHDR(1) + MAC payload +
+                       MIC(4), or MHDR(1) + FHDR(7) + FPort(1) + APP + MIC(4).
+                       For example, SIZE for Join Request is going to be 23.
+                       If the size of an application message (APP) is 12, SIZE
+                       is going to be 25.
 
 optional arguments:
   -h, --help           show this help message and exit
@@ -62,6 +66,7 @@ the --downlink option should be specified.
   --duty-cycle NUMBER  specify the duty cycle in percentage. default is 1 %.
   -v                   enable verbose mode.
   -d                   increase debug mode.
+```
 
 ## Examples
 
@@ -100,4 +105,3 @@ without the -v option, it simply shows the ToA.
 
 ![LoRa ToA](image/as923-toa.png)
 
-##