add cmake install instructions

README.rst

@@ -180,6 +180,16 @@ Detailed full installation instructions:
     Therefore, if the package is installed in a non standard location, it is recommanded
     to setup the environment variable: HDF5_ROOT.
 
+5.  TurTLE requires `cmake` version > 3.6 (possibly 3.12).
+    If required, download, compile and install cmake, currently
+    available at https://cmake.org/cmake/resources/software.html.
+
+    .. code:: bash
+
+        ./bootstrap --prefix=PREFIX
+        make
+        make install
+
 5.
     Optional.
     We recommend the creation of a virtual python3 environment (also under PREFIX) that will be used for installing TurTLE and dependencies.