join: add page (#2288)

pages/common/join.md

@@ -0,0 +1,15 @@
+# join
+
+> Join lines of two sorted files on a common field.
+
+- Join two files on the first (default) field:
+
+`join {{file1}} {{file2}}`
+
+- Join field3 of file1 with field1 of file2:
+
+`join -1 3 -2 1 {{file1}} {{file2}}`
+
+- Produce a line for each unpairable line for file1:
+
+`join -a 1 {{file1}} {{file2}}`