Clarify building from sources docs in README

This commit clarifies a few issues with the building from sources docs
in the README:
 - modifies Gradle link to use https
 - version 2.13 of Gradle is required
 - the command "gradle assemble" is the preferred way to only create a
   distribution
 - fixes the specified location of the built distributions

Closes #18991

README.textile

@@ -200,11 +200,11 @@ We have just covered a very small portion of what Elasticsearch is all about. Fo
 
 h3. Building from Source
 
-Elasticsearch uses "Gradle":http://gradle.org for its build system. You'll need to have a modern version of Gradle installed - 2.8 should do.
+Elasticsearch uses "Gradle":https://gradle.org for its build system. You'll need to have a modern version of Gradle installed - 2.13 should do.
 
-In order to create a distribution, simply run the @gradle build@ command in the cloned directory.
+In order to create a distribution, simply run the @gradle assemble@ command in the cloned directory.
 
-The distribution for each project will be created under the @target/releases@ directory in that project.
+The distribution for each project will be created under the @build/distributions@ directory in that project.
 
 See the "TESTING":TESTING.asciidoc file for more information about
 running the Elasticsearch test suite.