Add documentation

Signed-off-by: Knut Ahlers <knut@ahlers.me>

bin/git-credential-vault

@@ -1,6 +1,32 @@
 #!/usr/bin/env bash
 set -euo pipefail
 
+###
+# How to use
+###
+#
+# Create a key in Vault `secret/git-credential/<hostname>` so i.e.
+# `secret/git-credential/git.luzifer.io` and fill it with key-value
+# pairs according to the git-credential spec [1]
+#
+# So for example this could be the content:
+# {
+#   "protocol": "https",    # Force to use HTTPS
+#   "username": "luzifer",  # The username required for the host
+#   "password": "..."       # Password for the given user
+# }
+#
+# The `${key}=${value}` pairs are dumped as they are without further
+# processing. Therefore you can override all parameters the
+# git-credential spec allows you to set. I recommend to enforce the
+# `protocol` field to `https` as this helper is not checking the
+# protocol passed but only looks for the `host` value to fetch the
+# correct key from Vault.
+#
+# [1] https://git-scm.com/docs/git-credential
+#
+###
+
 source "${HOME}/bin/script_framework.sh"
 
 function handle_get() {