DOC: Update the release notes

RELEASE_NOTES.txt

+OTB-v 8.1.0 - Changes since version 8.0.1 (August 11th, 2022)
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+
+Features added:
+
+   * !917: Avoid reloading the same DEM directory by Julien Osman
+   * !904: Add default radius value to NewFunctorFilter overload by Laurențiu Nicola
+
+Bugs fixed:
+
+   * !922: Fix strange behavior of BandMath in python pipeline by calling RegisterPipeline() by Rémi Cresson
+   * !920: Correctly take into account the extended filename writerpctag by Julien Osman
+   * !918: Fix Pixel shift in pleiades orthorectification by Thibaut ROMAIN
+   * !914: Read RPC model from images writen by OTB by Julien Osman
+   * !879: Fix SetParameterString for listview parameters by Cédric Traizet
+
+Documentation:
+
+   * !889: HaralickTextureExtraction: Clarify the definition of the parameters xrad and yrad by Julien Osman
+   * !887: Improve Python errors of mismatched types by Luc Hermitte
+
+Licence and legislation matters:
+
+   * !905: Remove the 'Lena' image by Julien Osman
+
 OTB-v 8.0.1 - Changes since version 8.0.0 (April 27th, 2022)
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