revise guidelines start page

[joschirr]

• improve wording of first paragraph and update for the new environment and how stakeholders can contribute
• include file about H2020 OA guidelines compliancy
• update information about the OpenAIRE validator

[lnielsen]

I've made an update now and I think first two bullets have been addresses. I however still think we can improve the frontage a lot more, to make it much more clear what the OpenAIRE guidelines is.

Anyway, if you're ok with the changes, you can close this issue.