chromium: remove maintainer handles from README.md, mention electron

Specifically the maintainers section is quite outdated and prone to get
out of sync with whatever primary data we have (mostly meta.maintainers)
in each derivation.

In an attempt to lower the risk of ending up out of sync again, we
simply remove the maintainer handles.

Also adds a mention for the newly from source built electron variant, as
almost everything except `upstream-info.nix` bumps will trigger electron
rebuilds as well.

And lastly, removes mentions of `chromium{Beta,Dev}` and the
accompanying `google-chrome-{beta,dev}, that have been removed a few
months ago.

I might look into reworking bigger parts of the README.md in the future,
but this honestly isn't that high of a priority for me for now.

pkgs/applications/networking/browsers/chromium/README.md

@@ -1,26 +1,18 @@
 # Maintainers
 
 - Note: We could always use more contributors, testers, etc. E.g.:
-  - A dedicated maintainer for the NixOS stable channel
+  - Dedicated maintainers for the NixOS stable channel
   - PRs with cleanups, improvements, fixes, etc. (but please try to make reviews
     as easy as possible)
   - People who handle stale issues/PRs
-- Primary maintainer (responsible for all updates): @primeos
-- Testers (test all stable channel updates)
-  - `nixos-unstable`:
-    - `x86_64`: @danielfullmer
-    - `aarch64`: @thefloweringash
-  - Stable channel:
-    - `x86_64`: @Frostman
+
 - Other relevant packages:
-  - `chromiumBeta` and `chromiumDev`: For testing purposes only (not build on
-    Hydra). We use these channels for testing and to fix build errors in advance
-    so that `chromium` updates are trivial and can be merged fast.
-  - `google-chrome`, `google-chrome-beta`, `google-chrome-dev`: Updated via
-    Chromium's `upstream-info.nix`
-  - `ungoogled-chromium`: @squalus
+  - `google-chrome`: Updated via Chromium's `upstream-info.nix`.
+  - `ungoogled-chromium`: A patch set for Chromium, that has its own entry in Chromium's `upstream-info.nix`.
   - `chromedriver`: Updated via Chromium's `upstream-info.nix` and not built
-    from source.
+    from source. Must match Chromium's major version.
+  - `electron-source`: Various version of electron that are built from source using Chromium's
+    `-unwrapped` derivation, due to electron being based on Chromium.
 
 # Upstream links