README.md

@@ -1,2 +1,50 @@
 # prime-numbers-c
 
+Lists every prime number from 0 to 1,000,000 and reports how long the search
+took. It shows how fast a CPU is, but the result depends on how the program is
+written: algorithm, instruction set (here AVX2), compiler and flags.
+
+Example result on a 13th Gen Intel(R) Core(TM) i5-13600K:
+
+```text
+Found 78498 primes from 0 to 1000000 in 20.977 ms
+```
+
+## Build and run
+
+Needs `gcc` (Linux, or MinGW-w64 / MSYS2 on Windows), `make` and a CPU with AVX2.
+
+```bash
+make
+```
+
+```bash
+make run
+```
+
+```bash
+make clean
+```
+
+The executable is built into `dist/`. The primes go to standard output and the
+timing line to standard error, so this saves the list and still shows the time:
+
+```bash
+./dist/prime_avx2 > primes.txt
+```
+
+## Layout
+
+| Path | Content |
+| --- | --- |
+| `src/prime_avx2.c` | the program |
+| `Makefile` | build rules |
+| `docs/wiki/` | wiki pages, published to the Gitea wiki by `.gitea/workflows/publish-wiki.yml` |
+
+## Documentation
+
+See the wiki: Description, Results and Build and run.
+
+## License
+
+See [LICENSE](LICENSE).