chore: add upgrade guidelines

Author: dyladan

README.md

@@ -73,6 +73,12 @@ main();
 
 Because the npm installer and node module resolution algorithm could potentially allow two or more copies of any given package to exist within the same `node_modules` structure, the OpenTelemetry API takes advantage of a variable on the `global` object to store the global API. When an API method in the API package is called, it checks if this `global` API exists and proxies calls to it if and only if it is a compatible API version. This means if a package has a dependency on an OpenTelemetry API version which is not compatible with the API used by the end user, the package will receive a no-op implementation of the API.
 
+## Upgrade Guidelines
+
+- `1.0.0-rc.1`
+
+[#45](https://github.com/open-telemetry/opentelemetry-js-api/pull/45) `Span#context` renamed to `Span#spanContext`
+
 ## Useful links
 
 - For more information on OpenTelemetry, visit: <https://opentelemetry.io/>