Added a little bit more information on how to define custom build & i…

…nstall procedures in CONTRIBUTING.md

CONTRIBUTING.md

@@ -29,3 +29,35 @@ the ways might be adding some README or NOTES file into your `.agner` repo.
 Before commiting your agner.config, please make sure you ran `agner verify` on it so it passes at least
 some validity checks.
 
+Build command
+~~~~~~~~~~~~~
+
+If your target repository can't be built by simply invoking `rebar get-deps && rebar compile` then you need to supply
+a `build_command` property, for example:
+
+
+     {build_command, "make"}.
+
+The build command will be invoked in checked out directory containing target repository.
+
+Install command
+~~~~~~~~~~~~~~~
+
+If your target repository can't be built by simply invoking `rebar get-deps && rebar compile` then you need to supply
+a `install_command` property, for example:
+
+
+     {install_command, "make install"}.
+
+The install command will be invoked in checked out directory containing target repository. 
+
+
+Environment Variables
+~~~~~~~~~~~~~~~~~~~~~
+
+Here's the current list of environmentvariables that will be available to build & install commands:
+
+* AGNER_PREFIX -- A place where install procedure should consider putting installable files to. Right now it defaults to
+  `/usr/local` but eventually will point to a properly isolated deployment environment
+
+More variables to come later.