Merge pull request #1670 from NNPDF/add_version406_to_docs

add version 4.0.6 to releases list in docs

doc/sphinx/source/releases.rst

@@ -21,6 +21,9 @@ The code is tagged to contextualize the versioning, mark significant
 developments and to mark versions used to produce main results. The
 significant releases since the code was made public are:
 
+`Version 4.0.6 <https://github.com/NNPDF/nnpdf/releases/tag/4.0.6>`_
+    The last version that uses C++ objects in n3fit and validphys, and apfel for 
+    PDF evolution. 
 `Version 4.0.5 <https://github.com/NNPDF/nnpdf/releases/tag/4.0.5>`_
     The last version to support legacy genetic algorithms fits based on C++.
 `Version 4.0.4 <https://github.com/NNPDF/nnpdf/releases/tag/4.0.4>`_