docs: added plantuml generation script

docs/generate-uml.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+
+# This script uses plantuml to generate SVG images from inline plantuml descriptions.
+# See ./auth/oauth.md for an example.
+#
+# You need to have plantuml installed (brew install plantuml, or some other method).
+#
+# Either call directly to generate diagrams for all markdown files in this directory,
+# or add a --watch flag to rebuild SVGs on changes.
+
+cd "$( dirname "${BASH_SOURCE[0]}" )"
+
+if [[ "$1" == '--watch' ]]; then
+  npx --no-install nodemon --ext md --exec './generate-uml.sh'
+fi
+
+grep '@startuml' -rl --include '*.md' . | while read -r file ; do
+  echo "Generating : $file"
+  plantuml -tsvg "$file" 2> >(grep -v "CoreText note:")
+done