Added INSTALL instructions.

INSTALL.md

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+Building MsSpec_DFM
+===================
+
+To build MsSpec_DFM, you need a fortran compiler (gfortran), python > 3.6 and pip
+
+Getting the source code
+-----------------------
+
+To download the source code, you can clone the git repo of the project
+
+```
+$ git clone https://git.ipr.univ-rennes1.fr/epsi/MsSpec-DFM.git
+```
+
+If git is not avilable on your system, you can download a compressed archive
+on the [repo website](https://git.ipr.univ-rennes1.fr/epsi/MsSpec-DFM).
+Once downloaded, this archive should be uncompressed.
+
+
+Building and installing
+-----------------------
+
+```
+# Change to the src folder
+$ cd MsSpec-DFM/src
+
+# Type in 'make' or 'make all'
+$ make
+
+# This will install the commandline utility and python bindings
+$ make install
+```
+
+Every new Python packages and dependencies will be installed in the
+Python user site packages (under $HOME/.local/ tree).
+
+You can also use a Python virtual environment
+
+
+Uninstalling
+------------
+
+```
+$ pip uninstall msspec_dfm
+```