Polish README and repository presentation

[CompleteDotTech]

What changed

Adds a four-line "About this fork" block at the top of README.md, and nothing else. No upstream line is edited or removed. The note says this is a fork of ggml-org/llama.cpp, lists what the fork's master adds on top of upstream, and points readers to upstream for releases, docs and support.

What I verified

• The fork is 5 commits ahead and 2158 behind upstream (GitHub compare API). The commit titles are: fix Gemma 4 thinking generation prompt (touches the two Gemma 4 chat templates and tests/test-chat.cpp); audio transcription fallback for chat input (server); TurboQuant Vulkan turbo4 SET_ROWS support (plus a merge of its fix PR).
• GGML_TYPE_TURBO2_0, TURBO3_0 and TURBO4_0 appear in common/arg.cpp in the clone, and the transcription code exists in tools/server/server-context.cpp.
• mdcheck.py --http reports defects only in upstream's existing text (a malformed link, two 401 Hugging Face links, a hotlinked logo). I left those alone because the file is upstream's.

Not verified

• I did not build the fork or run its tests, so the note makes no claim that these changes work.
• The wording on the TurboQuant change is taken from commit titles and file lists, not from reading the implementation.

This PR is not to be auto-merged: it needs the owner's review.