Add: OpenTTD 1.9.0-RC1 news post

_posts/2019-03-03-1.9.0-RC1.md

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+---
+title: OpenTTD 1.9.0-RC1
+author: LordAro
+---
+
+OpenTTD 1.9.0-RC1 is now available, which means the 1.9 branch is now in feature freeze, so that's your lot in features!
+Nothing particularly huge since beta3, but full [changelog](https://proxy.binaries.openttd.org/openttd-releases/1.9.0-RC1/changelog.txt) and [download](https://www.openttd.org/downloads/openttd-releases/testing.html) can be found in the usual places
+
+Please help with testing, any issues can be reported in the other usual [place](https://github.com/OpenTTD/OpenTTD/issues)
+
+Enjoy!