Update README.md with v3.0.0-rc0 instructions

Adapt the README.md to the modern Gerrit v3.0.0 setup instructions
where ReviewDb is completely gone from everywhere, including the
gerrit.config.

Change-Id: I541754db8046ac052fd9faf4852355ab580a0af0
diff --git a/README.md b/README.md
index 004170d..597a0ea 100644
--- a/README.md
+++ b/README.md
@@ -28,12 +28,12 @@
 ## Build docker image
 
 For docker images that contain released Gerrit versions, tags exist in this git repository pointing
-to a state of the repository, where this version of Gerrit (e.g. 2.16.5) is referenced in the
+to a state of the repository, where this version of Gerrit (e.g. 3.0.0-rc0) is referenced in the
 Dockerfiles. To build such a docker image for development purposes, checkout the respective version
 tag, e.g.:
 
 ```
-git checkout v2.16.5
+git checkout v3.0.0-rc0
 ```
 
 Navigate to either `./centos/7` or `./ubuntu/16` to build the centos- or ubuntu-based docker image,
@@ -143,10 +143,6 @@
   basePath = git
   canonicalWebUrl = http://localhost
 
-[database]
-  type = h2
-  database = db/ReviewDB
-
 [index]
   type = LUCENE