Add section explaining backwards compatability and versioning plan

Signed-off-by: Chris Marchbanks <[email protected]>

Author: csmarchbanks

Diff for: proposals/2024-01-29_native_histograms_text_format.md

@@ -67,6 +67,10 @@ Positive infinity, negative infinity, and non number values will be represented
 
 Note that in this initial implementation float histograms are not supported.
 
+### Backwards compatability and semantic versioning
+
+After discussions with a few people it is believed that these changes can be made in a 1.x release of OpenMetrics. OpenMetrics 1.x parsers that support native histograms will still be able to read OpenMetrics 1.0 responses, therefore this change is backwards-compatible. However, this change is not forwards-compatible, i.e. an OpenMetrics 1.0 parser will not be able to read an OpenMetrics >= 1.1 response. Any producers implementing native histograms MUST also implement content negotiation and fall back to OpenMetrics 1.0.0, and therefore not expose native histograms, if a supported version cannot be negotiated. Note that the behavior to fall back to 1.0.0 is already part of the [OpenMetrics spec](https://github.com/OpenObservability/OpenMetrics/blob/main/specification/OpenMetrics.md#protocol-negotiation).
+
 ## Alternatives
 
 ### Do nothing