add repo description and build steps

README.md

@@ -1,7 +1,29 @@
+[Upstream README](README-upstream.md)
+
 # InChI unofficial repository
 
-This is just a fork of the [InChI library official repository](https://github.com/IUPAC-InChI/InChI) where I work on my changes before submitting them as PRs
+This is a fork of the [InChI library official repository](https://github.com/IUPAC-InChI/InChI) where I work on my changes before submitting them as PRs
 
 As such, there should be no need to use it unless you want to test some specific work in progress or artifact before it lands upstream
 
-[Upstream README](README-upstream.md)
+Right now, the main work going on here is the port of the build system to cmake (originally on branch `giallu/port_to_cmake`, later rebased and expanded on branch `build_matrix`):
+
+## Compiling from source
+
+InChI library and demo binaries can be easily compiled with cmake out of tree. 
+For instance, the following will build everything in the `builddir` directory:
+
+```
+cmake -B builddir
+cmake --build builddir
+```
+
+tested on Linux/GCC, Windows/MSVC, OSX/LLVM [and others](https://github.com/giallu/InChI/actions/workflows/build.yml)
+
+### Compilation options
+
+the default build creates a shared library, it is possible to have a static library instead by setting `BUILD_SHARED_LIBS` to `OFF` like this:
+
+```
+cmake -B builddir -D BUILD_SHARED_LIBS=OFF
+```