Update CONTRIBUTING.md

[benleamon]

2581 .

Changes proposed in this pull request:

• Adds information relating to contributions on behalf of employers, code coming from sources other than the contributor, and preserving copyright and attribution information for code coming from other projects.

@pgRouting/admins

[cvvergara]

Some suggestions that generated this PR

• Add information about contributions developed on behalf of an employer.
• Add information about contributions coming either from the contributor or another valid source (e.g. the public domain or a project with a compatible license).
• I noticed that before committing people are supposed to "Fill in the commit message template." I wasn't able to find this template, but it could be good to link to it in contributing.md
• Ben also noticed that there is another contributing.md file in .github.
• Should release processes and guidelines also be included here?
• OSGeo also recommends including expectations for committers