keep-header: add page (#8401)

pages/common/keep-header.md

@@ -0,0 +1,20 @@
+# keep-header
+
+> Keep first line untouched by a command, passing it directly to stdout.
+> More information: <https://github.com/eBay/tsv-utils#keep-header>.
+
+- Sort a file and keep the first line at the top:
+
+`keep-header {{path/to/file}} -- sort`
+
+- Output first line directly to stdout, passing the remainder of the file through the specified command:
+
+`keep-header {{path/to/file}} -- {{command}}`
+
+- Read from stdin, sorting all except the first line:
+
+`cat {{path/to/file}} | keep-header -- {{command}}`
+
+- Grep a file, keeping the first line regardless of the search pattern:
+
+`keep-header {{path/to/file}} -- grep {{pattern}}`