README.md updated

README.md

@@ -1,3 +1,20 @@
 # cawk
 
-REPL calculator in Bash and AWK
+REPL calculator in Bash and AWK
+
+```
+USAGE
+  cawk [-f PRECISION]                 - Start in REPL mode.
+  cawk -e EXPRESSION [-f PRECISION]   - Eval EXPRESSION and quit.
+  COMMAND | cawk [-f PRECISION]       - Eval expressions from COMMAND output.
+
+OPTIONS
+  -e EXPRESSION   Expression to evaluate.
+  -f PRECISION    Number of positions after decimal point.
+
+COMMANDS
+  f[0-9]    Redefine precision.
+  h|?       Help.
+  q|quit    Quit.
+```
+