Added guidance on raising issues

CONTRIBUTING.md

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+# Reporting bugs
+
+First, thanks for your interest in these example codes.
+
+The "Issues" system on this GitHub repository is intended for reporting bugs, errors, and other mistakes in these codes and their documentation.
+We welcome any such reports, and will endeavour to fix bugs as quickly as possible.
+
+Please feel free to open an issue in the following circumstances,
+assuming that you have already checked any current open issues, to avoid duplication.
+* You have discovered a definite bug or error in any of the Fortran or Python files. Please state clearly which file the error is in, and the nature of the error.
+* You have come across a mistake, or a broken link, in the documentation (the GUIDE.md files or the Wiki pages). Please specify exactly where the mistake is.
+
+
+Here are some examples of things _not_ to raise as issues.
+
+* Feature or enhancement requests. These example programs are solely intended to illustrate points made in the text.
+* General questions about molecular simulation methods. Discussions of this kind are outside the scope of this site.
+* Questions relating to building or running the codes on your specific computer system.
+
+Finally, please note that errata in the text of the book itself are summarized in a different repository: <https://github.com/Allen-Tildesley/corrections>. If you spot an error in the book, please report it there!