Add build instructions to README #163

Author: Toby McLaughlin

README.md

@@ -18,6 +18,20 @@ A full build and test requires:
 - GNU Make
 - Python 3.5 with Virtualenv
 
+## Running a build
+To build an image with the latest nightly snapshot of Elasticsearch, run:
+```
+make from-snapshot
+```
+
+To build an image with a released version of Elasticsearch, check out the corresponding
+branch for the version, and run Make while specifying the exact version desired.
+Like this:
+```
+git checkout 6.2
+ELASTIC_VERSION=6.2.4 make
+```
+
 ## Contributing, issues and testing
 
 Acceptance tests for the image are located in the test directory, and can be invoked with `make test`.