add description about a1 like notation

README.md

@@ -16,15 +16,76 @@ $ cj -s AA:AC,BC:CD input.csv > output.jsonl
 ```
 
 
+## installation
+
+download binary from [releases](https://github.com/musou1500/cj/releases) and place it in your PATH.
+
+
 ## A1 like notation
 
-A1 like notation is a way to specify columns in a CSV file.
+`-s` option is used to specify columns to convert in A1 like notation.
+but unlike A1 notation, you can not specify a range of rows. (e.g. 'A1:B2' is not supported)
 
-unlike A1 notation, you can not specify a range of rows. (e.g. 'A1:B2' is not supported)
+by using A1 like notation you can
 
-You can specify multiple ranges separated by commas.
+* select single column
+  * e.g. `A`
+* select range of columns
+  * e.g. `A:C` 
+* select multiple ranges of columns
+  * e.g. `A:C,E:G`
 
+in the following examples, notation is written in upper case. but it is case-insensitive.
 
-## installation
+suppose you have a CSV file like this.
 
-download binary from [releases](https://github.com/musou1500/cj/releases) and place it in your PATH.
+```csv
+A,B,C,...,Z,AA,AB,AC
+0,1,2,...,25,26,27,28
+```
+
+### select single column
+
+You can specify single column by sequence of alphabets.
+
+```bash
+$ cj -s AA input.csv # or `cj -s aa input.csv`
+["AA"]
+["26"]
+```
+
+### select range of columns
+
+You can select range of columns by following notation.
+
+```bash
+$ cj -s AA:AC input.csv
+["AA","AB","AC"]
+["26","27","28"]
+```
+
+If left side of colon is larger than right side, select columns in reverse order.
+
+```bash
+$ cj -s AC:AA input.csv
+["AC","AB","AA"]
+["28","27","26"]
+```
+
+### select multiple ranges of columns
+
+You can specify multiple ranges by separating them with comma.
+
+```bash
+$ cj -s A:B,D:E input.csv
+["A","B","D","E"]
+["0","1","3","4"]
+```
+
+overlapping ranges are supported. if then, concatenated columns are selected.
+
+```bash
+$ cj -s A:C,B:D ./input.csv
+["A","B","C","B","C","D"]
+["0","1","2","1","2","3"]
+```