explain the use of drawio to edit and publish diagrams

- where to store the original and svg final version
- installing drawio and aliasing the binary
- how to generate an SVG version of the diagram

README.md

@@ -60,6 +60,42 @@ Images to be included in the docs are kept in `source/images`
 
 In order to configure some aspects of layout, like the header, edit `config/tech-docs.yml`.
 
+## Making changes to a diagram
+To make changes to a diagram, you'll need to edit the `drawio` files in `source/images/originals` using the draw.io desktop app.
+
+Use one drawio file per diagram.
+
+### Install draw.io desktop app
+1. Install the draw.io desktop app using Homebrew with the following command: 
+
+```
+brew cask install drawio
+```
+
+2. Alias the binary so that you can just type `draw.io` at the command line. Use the following command to do this:
+```
+alias=draw.io='/Applications/draw.io.app/Contents/MacOS/draw.io'
+```
+
+### Edit and publish the diagram
+Follow these steps:
+1. Create and modify the diagram using [draw.io](https://about.draw.io/).
+1. Store the draw.io original file in `source/images/originals`.
+1. Publish the diagram as a [scalable vector graphic (SVG)](https://www.w3.org/Graphics/SVG/).
+1. Store the SVG in the `source/images` folder.
+
+### Useful commands
+Update a diagram:
+```
+draw.io source/images/originals/top-level-technical-diagram.drawio
+```
+Generate SVG versions of the diagrams:
+```
+draw.io -x -o source/images/top-level-technical-diagram.svg source/images/originals/top-level-technical-diagram.drawio
+draw.io -x -o source/images/technical-flow-diagram.svg source/images/originals/technical-flow-diagram.drawio
+```
+
+
 ### Workflow
 
 The repository uses Github actions.
@@ -145,4 +181,4 @@ The documentation is [© Crown copyright][copyright] and available under the ter
 
 [mit]: LICENCE.md
 [copyright]: http://www.nationalarchives.gov.uk/information-management/re-using-public-sector-information/uk-government-licensing-framework/crown-copyright/
-[ogl]: http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/
+[ogl]: http://www.nationalarchives.gov.uk/doc/open-government-licence/version/3/