Tools

GitHub

The general pattern is as follows:

Fork the original project's repository to have your own GitHub copy, to which you'll then be allowed to push changes.
Clone your GitHub repository onto your local machine
Optionally, add the original repository as an additional remote repository on your local repository. You'll then be able to fetch changes published in that repository directly.
Make your modifications and your own commits locally.
Push your changes to your GitHub repository (as you generally won't have the write permissions on the project's repository directly).
Contact the project's maintainers and ask them to fetch your changes and review/merge, and let them push back to the project's repository (if you and them want to).

PlantUML for documentation of flows, mostly as sequence diagrams There are a number of ways of viewing/authoring PlantUML (see downloads page).
The files can be rendered directly in a browser with no extensions using a proxy service with the URL format as follows http://www.plantuml.com/plantuml/proxy?fmt=svg&src=[sourcefile]: extension of the PlantUML proxy for SVG output, courtesy of Shane McCarron
Also a Chrome extension is available. This allows you when using Chrome to view a GitHub file to view the diagrams by simply clicking on the ‘Raw’ button For viewing local files, you will need to enable that in the extension (paste this link into Chrome chrome://extensions/ and tick ‘allow access to file URLs’)
If you have problems with using local tools for authoring (e.g. corporate policy), there is also a Web interface to author diagrams.