Add `jcamp.__version__` and document hints for contributing

[sumpfralle]

For various reasons it is helpful to offer a version string in order to let users work around incompatible changes (e.g. the recent renaming of JCAMP_* to jcamp_*.

The second commit tries to document the steps for creating a release.

Feel to free to comment on the contribution hints. These are just wild suggestions.

[sumpfralle]

Two more details:

• maybe you want to add the specific procedure for uploading a new version to PyPI in CONTRIBUTING.md?
• personally I prefer to use bumpversion for keeping the version string in sync across multiple files (but here in jcamp it still works with just these two relevant files containing the version string)

btw.: I think, you accidentally pushed the tag v0.9 for commit 5bf050af6f664b770bca43b4379bbc6c53ea1bce. You should probably discard it (git tag -d v0.9 && git push origin :v0.9). Currently it is the only release listed below releases here on github.

[nzhagen]

Thanks for the suggestions! Since this package has never been a part of my daily workflow, I've never taken the time to polish it up, and it has suffered from the neglect.