Add CONTRIBUTING.md files to main devfile repos

[elsony]

Which area/kind this issue is related to?
/area api
/area library

Issue Description

Add CONTRIBUTING.md files to the repo to help new contributors on developing and contribute code to the project.

This item covers the repos:

• https://github.com/devfile/api (** needed as part of CNCF onboarding)
• https://github.com/devfile/library
• https://github.com/devfile/landing-page
• https://github.com/devfile/devfile-web
• https://github.com/devfile/registry-support
• https://github.com/devfile/registry-viewer
• https://github.com/devfile/registry-operator

[yangcao77]

Library is done: devfile/library#135

[mike-hoang]

registry-support: devfile/registry-support#111
registry-operator: devfile/registry-operator#21

[michael-valdron]

registry-support is done: devfile/registry-support#111