Add guidelines for naming link types

[magnusbaeck]

Applicable Issues

Fixes #231

Description of the Change

This puts in writing what has been the de facto naming standard for all current event types, namely that they should be nouns and fit one of these patterns:

<target> is the <source>'s <type>
<target> is one of the <source>'s <type>s

Alternate Designs

None. This design rule isn't really novel, it's just putting words to today's de facto standard.

Benefits

This should make it easier to come up with good and consistent names of link types that we consider adding.

Possible Drawbacks

None.

Sign-off

Developer's Certificate of Origin 1.1

By making a contribution to this project, I certify that:

(a) The contribution was created in whole or in part by me and I have the right to submit it under the open source license indicated in the file; or

(b) The contribution is based upon previous work that, to the best of my knowledge, is covered under an appropriate open source license and I have the right under that license to submit that work with modifications, whether created in whole or in part by me, under the same open source license (unless I am permitted to submit under a different license), as indicated in the file; or

(c) The contribution was provided directly to me by some other person who certified (a), (b) or (c) and I have not modified it.

(d) I understand and agree that this project and the contribution are public and that a record of the contribution (including all personal information I submit with it, including my sign-off) is maintained indefinitely and may be redistributed consistent with this project or the open source license(s) involved.

Signed-off-by: Magnus Bäck <magnus.back@axis.com>

[e-backmark-ericsson]

Looks good to me.
And I don't think we need a link here from https://github.com/eiffel-community/eiffel/blob/master/eiffel-syntax-and-usage/the-links-object.md. Having such a link there would add the requirement for similar links in many other places as well.