Remove deprecated fetch_surf_fsaverage5 (nilearn#2447)

* Update struct.py

Issue nilearn#2358

* Update CONTRIBUTING.rst to include developer install instructions

As a newcomer I had difficulty figuring how to install the nilearn package so that my code  changes were reflected immediately. This may help others like me. Let me know if you think it's useful and feel free to suggest changes to wording :) .

CONTRIBUTING.rst

@@ -61,6 +61,14 @@ You can check out the latest sources with the command::
 or if you have write privileges::
 
     git clone [email protected]:nilearn/nilearn.git
+    
+Installing the latest code
+---------------------------
+
+In order to ensure that any code changes are reflected in your installation, navigate to your cloned Nilearn base directory and install using the following command::
+
+    pip install -e .
+
 
 Coding guidelines
 ------------------

nilearn/datasets/struct.py

@@ -500,9 +500,8 @@ def _fetch_surf_fsaverage(data_dir=None):
 
 
 def fetch_surf_fsaverage5(data_dir=None, url=None, resume=True, verbose=1):
-    """ Deprecated since version 0.4.3
-
-    Use fetch_surf_fsaverage instead.
+    """  NOTE: This function is deprecated and will be removed in
+    0.7.0 release. Use `fetch_surf_fsaverage` instead.
 
     Parameters
     ----------
@@ -529,7 +528,7 @@ def fetch_surf_fsaverage5(data_dir=None, url=None, resume=True, verbose=1):
 
     """
     warnings.warn("fetch_surf_fsaverage5 has been deprecated and will "
-                  "be removed in a future release. "
+                  "be removed in 0.7.0 release. "
                   "Use fetch_surf_fsaverage(mesh='fsaverage5')",
                   np.VisibleDeprecationWarning, stacklevel=2)
     return fetch_surf_fsaverage(mesh='fsaverage5', data_dir=data_dir)