add local-dev/README.md

Author: ido

.gitignore

@@ -1,3 +1,4 @@
 dist/
 .jenkins
-deploy/spinnaker/
+deploy/spinnaker/
+/local-dev/PreConfig.js

local-dev/README.md

@@ -1,3 +1,20 @@
 This directory is just for local development purpose.
 Originally copied from debian/*.
 
+## Additional configuration
+
+Addition to [../README](../README.md), the following variables can be
+customized:
+
+* **SERVER_URL_PREFIX**:  base URL for this server
+* **EDITOR_UI_DRAW_URL**: viewer URL for a file saved as *.html extention
+
+## How to build
+
+1. customize PreConfig.js.  It is git-ignored.
+2. docker build
+      > $ docker build -t <tag> .
+
+## Reference
+
+* <https://github.com/maedadev/drawio>