[Documentation] dist/README Chromium manual install: clarify MV2 (uBlock0.chromium) vs MV3-only browsers

### Type **Documentation** — not a uBlock Origin code bug. ### Summary Following [README → Manual Installation → dist#chromium](https://github.com/gorhill/uBlock/tree/master/dist#chromium) on **Fedora 44 + Flatpak Chromium 148** leads users to load `uBlock0.chromium` (Manifest **V2**). Chromium **148** is **MV3-only**, so the browser correctly rejects the extension: ```text Failed to load extension Cannot install extension because it uses an unsupported manifest version. Could not load manifest. ``` The root README table mentions *"End of support on Chrome 139"* for the store row, but the **manual install page does not explain MV2 vs MV3**, what `uBlock0.chromium` is, or what to do on current Chromium. That gap caused this report to be filed as a product bug. ### What we tried (repro context) - Release [uBlock0_1.71.0.chromium.zip](https://github.com/gorhill/uBlock/releases/download/1.71.0/uBlock0_1.71.0.chromium.zip) — `manifest_version: 2` - `make chromium` on `master` — same MV2 output - Browser: Chromium **148.0.7778.96** via Flatpak `org.chromium.Chromium` on Fedora 44 KDE Plasma ### Requested change Clarify in **`dist/README.md` (Chromium / manual install)** and cross-link from **README.md (Chromium section)**: 1. GitHub releases + `make chromium` = **MV2** (`uBlock0.chromium`). 2. Chromium **~139+** rejects MV2 → the error above is **expected**, not a corrupt build. 3. Pointers for current Chromium users: Web Store (if available), **Firefox** for full uBO, or **`make mv3-chromium`** / **uBO Lite** for unpacked MV3 (separate product; [uBOL tracker](https://github.com/uBlockOrigin/uBOL-home/issues)). ### Proposed doc patch (local draft) Reporter prepared wording in a fork/clone; happy to open a PR to `gorhill/uBlock` if maintainers want it. Key addition is a **"Manifest V2 vs Manifest V3 (read this first)"** section at the top of `dist/README.md#chromium`. ### Related - [#3563](https://github.com/uBlockOrigin/uBlock-issues/issues/3563) — MV2 removal / Chrome "no longer supported" - [#3860](https://github.com/uBlockOrigin/uBlock-issues/issues/3860) — same error on Canary (closed as external/invalid; same underlying MV2 vs MV3 mismatch) ### Environment (for reproducibility only) | Item | Value | |------|--------| | OS | Fedora Linux 44 (KDE Plasma) | | Browser | Chromium 148.0.7778.96 (Flatpak `org.chromium.Chromium`) | Screenshot of the Chromium error dialog available on request (reporter can attach).