Adding contributing guidelines regarding specs

For developers, calling out `rake spec:show_matchers` as a potentially
helpful tool.

Also adjusting `rake spec:show_matchers` so that it properly runs.

HYDRASIR-26

[skip ci]

JIRA References:
https://jira.duraspace.org/browse/HYDRASIR-26
Merge pull request #399 from projecthydra/feature/verifying-rake-spec-show_matchers

Adding contributing guidelines regarding specs