Add contributing guidelines to README

Added contributing section with guidelines for contributions.

Author: cortner

README.md

@@ -7,3 +7,7 @@
 The `ACEhamiltonians` package is a `Julia` package that provides tools for constructing, fitting, and predicting self-consistent Hamiltonian and overlap matrices in solid-state systems. It is based on the atomic cluster expansion (ACE) approach and the associated [ACEsuit package](https://github.com/ACEsuit/ACE.jl). The `ACEhamiltonians` package contains functions for generating on-site and off-site basis functions, fitting these bases to theoretical (DFT) data, and predicting the Hamiltonian and overlap matrices for any atomic configuration in real or reciprocal space. `ACEhamiltonians` provides a flexible and efficient way to model the electronic structure of materials and is a valuable tool for researchers in computational materials science. Please refer to the associated [article](https://www.nature.com/articles/s41524-022-00843-2) for a more in-depth description of the methodological underpinnings of this package.
 
 Please consult the [quick start guide](https://ACEsuit.github.io/ACEhamiltonians.jl/dev/) for installation and usage instructions.
+
+## Contributing
+
+Contributions are very welcome. Until clear guidelines and practices are established, we recommend to open an issue where the bugfix or enhancement can be discussed, before starting a pull request. We will do our best to respond in a timely manner.