Add install from source instructions

Author: mhvk

quaddtype/README.md

@@ -20,3 +20,36 @@ np.array([1,2,3], dtype=QuadPrecDType("sleef"))
 # using longdouble backend
 np.array([1,2,3], dtype=QuadPrecDType("longdouble"))
 ```
+
+## Install from source
+
+The code needs the quad precision pieces of the sleef library, which
+is not available on most systems by default, so we have to generate
+that first.  The below assumes one has the required pieces to build
+sleef (cmake and libmpft-dev), and that one is in the package
+directory locally.
+
+```
+git clone https://github.com/shibatch/sleef.git
+cd sleef
+cmake -S . -B build -DSLEEF_BUILD_QUAD:BOOL=ON -DSLEEF_BUILD_SHARED_LIBS:BOOL=ON -DCMAKE_POSITION_INDEPENDENT_CODE=ON
+cmake --build build/ --clean-first -j
+cd ..
+```
+
+In principle, one can now install this system-wide, but easier would
+seem to use the version that was just created, as follows:
+```
+export SLEEF_DIR=$PWD/sleef/build
+python3 -m venv temp
+source temp/bin/activate
+pip install meson-python numpy pytest
+DFLAGS="-Wl,-rpath,$SLEEFDIR/lib -I$SLEEF_DIR/include" python -m pip install -e . -v --no-build-isolation
+export LD_LIBRARY_PATH=SLEEF_DIR/lib
+```
+
+Here, we created an editable install on purpose, so one can just work
+from the package directory if needed, e.g., to run the tests with,
+```
+python -m pytest
+```