Add description of file format to README

README.md

@@ -1,2 +1,35 @@
 # Sudoku solver
 A simple sudoku problem solver
+
+## File format
+The sudoku solver accepts a file for the following format:
+* Each row consists of 9 characters (+ newline)
+* Each character on a row (aside from newline) must be '0'-'9' or ' ' (if value is unknown)
+* There must be a total of 9 rows
+
+For example, a file might look as follows:
+```
+ 23456789
+2 4567891
+34 678912
+456 89123
+5678 1234
+67891 345
+789123 56
+8912345 7
+91234567 
+```
+
+or
+
+```
+123456789
+2 456 891
+3456789  
+456 89123
+5678 12 4
+6  91 345
+789123456
+8 12345 7
+912345678
+```