Create .gitignore for convenient contributing

Currently, if someone compiled the programs in src and then *.o, *.so were produced, he cannot send a merge request in the current repository because *.o files will make the remote not clean. Also, if src/Makefile is modified, merge request cannot be sent because of conflict. So, a .gitignore is added to prevent such conditions, which ignores *.o, *.so, .mod, .make. A detailed explanation is available in the CONTRIBUTING.md.