Put skggm as optional

Author: William de Vazelhes

README.rst

@@ -20,8 +20,13 @@ Metric Learning algorithms in Python.
 **Dependencies**
 
 -  Python 2.7+, 3.4+
--  numpy, scipy, scikit-learn, and skggm (commit `a0ed406 <https://github.com/skggm/skggm/commit/a0ed406586c4364ea3297a658f415e13b5cbdaf8>`_) for `SDML`
--  (for running the examples only: matplotlib)
+-  numpy, scipy, scikit-learn
+
+**Optional dependencies**
+
+- For SDML, convergence will be better in some cases with skggm
+(install from commit `a0ed406 <https://github.com/skggm/skggm/commit/a0ed406586c4364ea3297a658f415e13b5cbdaf8>`_).
+-  For running the examples only: matplotlib
 
 **Installation/Setup**
 

doc/getting_started.rst

@@ -15,8 +15,13 @@ Alternately, download the source repository and run:
 **Dependencies**
 
 -  Python 2.7+, 3.4+
--  numpy, scipy, scikit-learn, and skggm (commit `a0ed406 <https://github.com/skggm/skggm/commit/a0ed406586c4364ea3297a658f415e13b5cbdaf8>`_) for `SDML`
--  (for running the examples only: matplotlib)
+-  numpy, scipy, scikit-learn
+
+**Optional dependencies**
+
+- For SDML, convergence will be better in some cases with skggm
+(install from commit `a0ed406 <https://github.com/skggm/skggm/commit/a0ed406586c4364ea3297a658f415e13b5cbdaf8>`_).
+-  For running the examples only: matplotlib
 
 **Notes**