Create README.md

Author: thofma

README.md

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+# libsingular-julia
+
+This is the C++ library accompanying [Singular.jl](https://github.com/oscar-system/Singular.jl).
+It implements the C++ interface from julia to Singular using [CxxWrap.jl](https://github.com/JuliaInterop/CxxWrap.jl) and [libcxxwrap-julia](https://github.com/JuliaInterop/libcxxwrap-julia).
+
+For Singular.jl versions less than 0.3.2 this was included in Singular.jl but version 0.4 will probably use this library as a separate artifact.
+
+## Building
+
+Compiling `libsingular-julia` from source requires a C++ enabled compiler, a `libcxxwrap-julia` installation and a Singular installation.
+
+```bash
+git clone https://github.com/oscar-system/libsingular-julia \
+cmake -DJulia_PREFIX=/home/user/path/to/julia \
+      -DSingular_PREFIX=/home/user/path/to/singular
+      -DCMAKE_INSTALL_PREFIX=home/user/prefix/for/libsingular-julia \
+      -DJlCxx_DIR=/home/user/path/to/libcxxwrap-julia/lib/cmake/JlCxx \
+      -DCMAKE_BUILD_TYPE=Release \
+      -S libsingular-julia -B build \
+
+cmake --build build --config Release --target install -- -j${nproc}
+```
+
+### Overriding the default artifacts for Singular.jl
+
+Put the following into `~/.julia/artifacts/Overrides.toml` to replace the `libsingular-julia` artifact:
+
+```toml
+[ae4fbd8f-ecdb-54f8-bbce-35570499b30e]
+libingular_julia = "/home/user/prefix/for/libsingular-julia"
+```
+
+Overrides for `Singular` and `libcxxwrap-julia` with the directories used during the build need to be added as well, e.g.:
+
+```toml
+[bcd08a7b-43d2-5ff7-b6d4-c458787f915c]
+Singular = "/home/user/path/to/Singular"
+
+[3eaa8342-bff7-56a5-9981-c04077f7cee7]
+libcxxwrap_julia = "/home/user/path/to/libcxxwrap-julia"
+```