Improve `CONTRIBUTING.md` and add `code_style.md` (#4708)
* Fix typo in README
* Add proper contributing guide
This is based on the same in element-web repo but with the following
changes:
1. Uses sign-off instead of CLA
2. Removes react, app specific instructions eg: tests do not mention
playwright.
* Add code_style.md
Copied from element-web repo but react/css specific items have been
removed.
* Fix lint
