feat(dev): add devcontainer config, add instructions to README

[tfenster]

I wanted to create an APK for testing and prefer devcontainers (https://code.visualstudio.com/docs/devcontainers/containers) as they allow me to keep dev environments separated and uncluttered, I did two things:

• I added the devcontainer.json file to enable that part. It looks like you are using prettier, so I added that extension, but others would be possible as well
• I extended the README to include the devcontainer explanation and a step by step explanation for getting an Android apk

I tested npm run / npm run web, which works fine, but I can't test the Android and iOS emulators as I don't have them (and to be honest, I kind of doubt this will work from a devcontainer). And I ran an Android apk build, which worked perfectly fine, allowing me now to have evcc as an app on my mobile :)

Please let me know if you want any adjustments. Also, if you disagree with the approach, I can just keep my fork and would of course accept that

Thanks for your work!

[tfenster]

not sure why this shows the whole file as changed, but selecting the "Display the rich diff" option shows the actual changes

[tfenster]

It would even be possible to automatically spin up an evcc container in demo mode next to it and connect to it. Whoever when I try to do that and the web app is running on http://localhost:8081 and the demo evcc container is running on http://localhost:7070, I get a CORS error:

If we could fix that (and I don't know how), then creating a development environment for the web app connected to a demo container could be done in a few minutes without additional knowledge, lowering the barrier for others to contribute. If you would like to investigate further, I'd be more than happy to help.