gh-2043: document 'gopass cat' (#2051)

RELEASE_NOTES=[DOCUMENTATION] document 'gopass cat'

Signed-off-by: Thomas Mantl <thomas.mantl@redgears.net>

docs/commands/cat.md

@@ -0,0 +1,47 @@
+# `cat` command
+
+The `cat` command is used to pipe password in and out of STDIN and STDOUT
+respectively. As it is intended to be used with binary data, it encodes the
+data-stream to store it.
+
+## Synopsis
+
+```bash
+$ echo "test" | gopass cat test/new
+$ gopass cat test/new
+```
+
+## Modes of operation
+
+* Create a new entry with data-stream from STDIN
+* Change an existing entry to data-stream from STDIN
+* Retrive encoded data from password-store and echo it to STDOUT
+
+Cat is intended to work with binary data, so it accepts any kind of stream from
+STDIN. It reads the binary-stream from STDIN and encodes it Base64 and saves it
+in the password store encoded, with some metadata about the input-stream and the
+used encoding (currently only Base64 supported).
+
+### Example
+```
+$ echo "234" | gopass cat test/new
+$ gopass show -f test/new
+Secret: test/new
+
+
+content-disposition: attachment; filename="STDIN"
+content-transfer-encoding: Base64
+MjM0Cg==
+$ gopass cat test/new
+234
+```
+
+### Differences to `insert`
+
+In contrast to `insert` it handles any kind of data-stream from STDIN and
+encodes it.
+Drawback: you can not just simply read the password with `gopass show`.
+
+## Flags
+
+This command has currently no supported flags except the gopass globals.