docs: document removal of shutdown flag in OTLPExporterBase (#2807)

Co-authored-by: Gerhard Stöbich <deb2001-github@yahoo.de>
Co-authored-by: Daniel Dyla <dyladan@users.noreply.github.com>
Co-authored-by: Valentin Marchaud <contact@vmarchaud.fr>

README.md

@@ -275,6 +275,10 @@ These instrumentations are hosted at <https://github.com/open-telemetry/opentele
 
 ## Upgrade guidelines
 
+### 0.27.x to 1.x.y
+
+- In `@opentelemetry/exporter-trace-otlp-http`, `OTLPExporterBase._isShutdown` is replaced with `_shutdownOnce`.
+
 ### 0.26.x to 0.27.x
 
 Metric and trace exporters are split into separate packages: