master [TASK][DOC]: describe make variables being used

INSTALL.md

+How to install Software for Seismometry (Linux)
+===============================================
+
+Quick guide
+-----------
+Compilation for some of the programs is supported by a Makefile.
+Just run
+
+    make all
+
+Detailed instructions
+---------------------
+The main [Makefile](Makefile) delegates compilation to 
+[software/Makefile](software/Makefile), where rules for the compilation and
+installation of binary executables and additional shell scripts are setup.
+The rules make use of some environment variables in order to find the location
+where files are to be installed and to identify local compilers.
+They are a subset of the variables used by
+[Seitosh](http://git.scc.kit.edu/Seitosh/Seitosh).
+Defaults are set in [Makefile](Makefile) and can be overwritten by variables
+exported from the shell's environment.
+
+### make variables
+Currently the Makefile sets defaults for three variables.
+
+#### `LOCBINDIR`
+Defines the destination path for the installation of binary executables.
+The default is directory `bin` in the projects root directory.
+
+#### `LOCSCRIPTDIR`
+Defines the destination path for the installation of shell scripts.
+The default is directory `bin` in the projects root directory.
+
+#### `FC`
+Defines the Fortran compiler to be used.
+The default is `gfortran`.