Config.exs: Add big warning so that nobody ever edits it.

config/config.exs

+#                                 .i;;;;i.
+#                               iYcviii;vXY:
+#                             .YXi       .i1c.
+#                            .YC.     .    in7.
+#                           .vc.   ......   ;1c.
+#                           i7,   ..        .;1;
+#                          i7,   .. ...      .Y1i
+#                         ,7v     .6MMM@;     .YX,
+#                        .7;.   ..IMMMMMM1     :t7.
+#                       .;Y.     ;$MMMMMM9.     :tc.
+#                       vY.   .. .nMMM@MMU.      ;1v.
+#                      i7i   ...  .#MM@M@C. .....:71i
+#                     it:   ....   $MMM@9;.,i;;;i,;tti
+#                    :t7.  .....   0MMMWv.,iii:::,,;St.
+#                   .nC.   .....   IMMMQ..,::::::,.,czX.
+#                  .ct:   ....... .ZMMMI..,:::::::,,:76Y.
+#                  c2:   ......,i..Y$M@t..:::::::,,..inZY
+#                 vov   ......:ii..c$MBc..,,,,,,,,,,..iI9i
+#                i9Y   ......iii:..7@MA,..,,,,,,,,,....;AA:
+#               iIS.  ......:ii::..;@MI....,............;Ez.
+#              .I9.  ......:i::::...8M1..................C0z.
+#             .z9;  ......:i::::,.. .i:...................zWX.
+#             vbv  ......,i::::,,.      ................. :AQY
+#            c6Y.  .,...,::::,,..:t0@@QY. ................ :8bi
+#           :6S. ..,,...,:::,,,..EMMMMMMI. ............... .;bZ,
+#          :6o,  .,,,,..:::,,,..i#MMMMMM#v.................  YW2.
+#         .n8i ..,,,,,,,::,,,,.. tMMMMM@C:.................. .1Wn
+#         7Uc. .:::,,,,,::,,,,..   i1t;,..................... .UEi
+#         7C...::::::::::::,,,,..        ....................  vSi.
+#         ;1;...,,::::::,.........       ..................    Yz:
+#          v97,.........                                     .voC.
+#           izAotX7777777777777777777777777777777777777777Y7n92:
+#             .;CoIIIIIUAA666666699999ZZZZZZZZZZZZZZZZZZZZ6ov.
+#
+#                          !!! ATTENTION !!!
+# DO NOT EDIT THIS FILE! THIS FILE CONTAINS THE DEFAULT VALUES FOR THE CON-
+# FIGURATION! EDIT YOUR SECRET FILE (either prod.secret.exs, dev.secret.exs).
+#
 # This file is responsible for configuring your application
 # and its dependencies with the aid of the Mix.Config module.
 #