Update documentation for building ELPA from git version

INSTALL

@@ -11,6 +11,11 @@ If you want to build (or have to since no packages are available) ELPA yourself,
 please note that ELPA is shipped with a typical "configure" and "make"
 procedure. This is the only supported way how to build and install ELPA.
 
+If you obtained ELPA from the official git repository, you will not find
+the needed configure script! Please look at the "INSTALL_FROM_GIT_VERSION" file
+for the documentation how to proceed.
+
+
 If --- against our recommendations --- you do not want to install ELPA as
 library, but to include it in your source code you will have to find a solution
 by yourself. If you do this anyway, please distribute then all files of ELPA

INSTALL_FROM_GIT_VERSION

+Welcome to the git-based distribution of the ELPA eigensolver library.
+
+If you are reading this file, you have obtained the ELPA library
+through the git repository that hosts the source code and also allows
+you to contribute improvements to the project if necessary.
+
+The git version does not contain the necessary build script:
+configure, Makefile ...
+
+If you use the git version, you are most likely actively developing
+features/improvements for ELPA, and a rebuild of the autotools scripts
+will be necessary anyway.
+
+Thus please run "autogen.sh" after your changes, in order to build the
+autotoos scripts. Note that autoconf version >= 2.69 is needed for
+ELPA.
+
+After this step, please proceed as written in the "INSTALL" file.
+

README

-Welcome to the git-based distribution of the ELPA eigensolver library.
-
-If you are reading this file, you have obtained the ELPA library
-through the git repository that hosts the source code and also allows
-you to contribute improvements to the project if necessary.
+If you obtained ELPA via the official git repository please have
+a look at the "INSTALL_FROM_GIT_VERSION" for specific instructions
 
 In your use of ELPA, please respect the copyright restrictions
 found below and in the "COPYING" directory in this repository. In a