add base64 page for linux

pages/linux/base64.md

@@ -0,0 +1,19 @@
+# base64
+
+> Encode or decode file or standard input, to standard output.
+
+- Encode a file
+
+`base64 {{filename}}`
+
+- Decode a file
+
+`base64 -d {{filename}}`
+
+- Encode from stdin
+
+`{{somecommand}} | base64`
+
+- Decode from stdin
+
+`{{somecommand}} | base64 -d`