RELEASE_NOTES: update for RC1.

RELEASE_NOTES.md

@@ -1,16 +1,16 @@
-we're proud to announce the first release candidate for the upcoming 2.6 series of darktable, 2.6.0rc0!
+we're proud to announce the second release candidate for the upcoming 2.6 series of darktable, 2.6.0rc1!
 
-the github release is here: [https://github.com/darktable-org/darktable/releases/tag/release-2.6.0rc0](https://github.com/darktable-org/darktable/releases/tag/release-2.6.0rc0).
+the github release is here: [https://github.com/darktable-org/darktable/releases/tag/release-2.6.0rc1](https://github.com/darktable-org/darktable/releases/tag/release-2.6.0rc1).
 
 as always, please don't use the autogenerated tarball provided by github, but only our tar.xz. the checksums are:
 
 ```
-$ sha256sum darktable-2.6.0~rc0.tar.xz
-5317f6353a1811ffc1e4c06fb983db5cd0bcfdccd6d8f595f470a3536424658f darktable-2.6.0rc0.tar.xz
-$ sha256sum darktable-2.6.0rc0.dmg
-??? darktable-2.6.0rc0.dmg
-$ sha256sum darktable-2.6.0rc0.exe
-??? darktable-2.6.0rc0.exe
+$ sha256sum darktable-2.6.0~rc1.tar.xz
+202bb53e924429aec74cd0a864b3d6a5c4d57b54547ef858bbd253116b909d22 darktable-2.6.0rc1.tar.xz
+$ sha256sum darktable-2.6.0rc1.dmg
+??? darktable-2.6.0rc1.dmg
+$ sha256sum darktable-2.6.0rc1.exe
+??? darktable-2.6.0rc1.exe
 ```
 
 when updating from the currently stable 2.4.x series, please bear in mind that your edits will be preserved during this process, but it will not be possible to downgrade from 2.6 to 2.4.x any more.