Add comment to README giving instructions on how to install to a directory other than /usr/local (#694)

Author: astrobit

Diff for: README.md

@@ -34,6 +34,8 @@ format to store user input files.
 Thanks to David Seifert (@SoapGentoo), we (the maintainers) now use [meson](http://mesonbuild.com/) and [ninja](https://ninja-build.org/) to build for debugging, as well as for continuous integration (see [`travis.sh`](travis.sh) ). Other systems may work, but minor things like version strings might break.
 
 First, install both meson (which requires Python3) and ninja.
+If you wish to install to a directory other than /usr/local, set an environment variable called DESTDIR with the desired path:
+    DESTDIR=/path/to/install/dir
 
 Then,