Clarify compatibility and content negotiation

[krajorama]

Following the dev summit on 13th September in Berlin.

https://docs.google.com/document/d/1uurQCi5iVufhYHGlBZ8mJMK_freDFKPG0iYBQqJ9fvA/edit#bookmark=id.kj599un8yoj7

From the dev summit document:

CONSENSUS: We intend to add native histograms as a part of OpenMetrics 1.1. A major version bump is not required.
CONSENSUS: We agree that the understanding of backwards compatibility is that a A.y parser can also parse an A.x format. But the A.x parser doesn’t need to parse the A.y format. Where y > x, A, x, y being integers
CONSENSUS: OpenMetrics doesn’t need a major version bump if the changes are backwards compatible.

Notes:

• this is my understanding
• I'm suggesting a more comprehensive update than agreed in the dev summit
• I think the document should actually state its own version (1.0.0) , but I'll leave that to a different PR

[krajorama]

@RichiH I've moved this PR to draft, because of two things:

1. Now it doesn't say explicitly what the dev summit agreed on: "CONSENSUS: We agree that the understanding of backwards compatibility is that a A.y parser can also parse an A.x format. But the A.x parser doesn’t need to parse the A.y format. Where y > x, A, x, y being integers." In particular if something as backwards compatible, then my originally suggested statement that parser for 1.5 must be able to parse 1.0, 1.1, 1.2, 1.4 as well stands.
2. Not sure what to do with the specification/draft-richih-opsawg-openmetrics-00.txt and specification/draft-richih-opsawg-openmetrics-01.txt files.