Update README.md

Author: pprcht

README.md

@@ -2,8 +2,60 @@
 # GFN0-xTB
 
 This repository contains a standalone implementation of
-the GFN0-xTB method (<https://doi.org/10.26434/chemrxiv.8326202.v1>).
+the GFN0-xTB method (<https://doi.org/10.26434/chemrxiv.8326202.v1>), 
+adapted from the [`xtb` code](https://github.com/grimme-lab/xtb).
 
 The default CMake and meson builds compile a statically linked library (`libgfn0.a`) that can be linked in other projects.
 
-`main.f90` in `testprog/` demonstrates the in-code usage.
+[`main.f90`](testprog/main.f90) in [`testprog/`](testprog/) demonstrates the in-code usage.
+
+
+---
+
+## Building the Project
+
+Make sure you have the following dependencies installed:
+
+- CMake and `make`, or meson and ninja build systems
+- Fortran and C compilers (e.g., `gfortran`/`gcc` or `ifort`/`icc`)
+
+### Instructions
+
+Follow these steps to build the project:
+
+1. Create a build directory and navigate to it
+   ```bash
+   mkdir _build
+   cd _build
+   ```
+
+2. Export the compilers (here for example `ifort`/`icc`) and depending on your chosen build system   set up the build:
+   - generate the build files using CMake:
+     ```bash
+     FC=ifort CC=icc cmake ..
+     ```
+   - generate the build files using meson:
+     ```bash
+     FC=ifort CC=icc meson ..
+     ```
+   I you wish to build the test-binary, add `-Dbuild_exe=true` to either the `cmake` or `meson`      setup command.
+
+
+3. Depending on your chosen build system, build the project. If you have multiple cores/processors,  you can speed up the build process by specifying the number of cores to use with the `-j` option.    For example, to use 4 cores:
+   - With CMake/`make`:
+     ```shell
+     make -j4
+     ```
+   - With meson/`ninja`:
+     ```shell
+     ninja -j4
+     ```
+### Cleaning the Build
+
+To clean the build files, simply delete the `build` directory:
+
+```shell
+rm -rf _build
+```
+
+