add comm command

pages/common/comm.md

@@ -0,0 +1,23 @@
+# comm
+
+> Select or reject lines common to two files. Both files must be sorted.
+
+- Produce three tab-separated columns: lines only in first file, lines only in second file and common lines.
+
+`comm {{file1}} {{file2}}`
+
+- Print only lines common to both files.
+
+`comm -12 {{file1}} {{file2}}`
+
+- Print only lines common to both files, read one file from stdin.
+
+`cat {{file1}} | comm -12 - {{file2}}`
+
+- Print lines only found in first file.
+
+`comm -23 {{file1}} {{file2}}`
+
+- Print lines only found in second file.
+
+`comm -13 {{file1}} {{file2}}`