Add note about cache dir need to be writable by server user

As per
https://groups.google.com/d/msg/cartodb/z06r9SwaoOM/b34In4TTdd0J

config/environments/development.js.example

@@ -36,6 +36,7 @@ var config = {
       bufferSize: 64
     }
     ,millstone: {
+        // Needs to be writable by server user
         cache_basedir: '/tmp/cdb-tiler-dev/millstone-dev'
     }
     ,redis: {

config/environments/production.js.example

@@ -30,6 +30,7 @@ var config = {
       bufferSize: 64
     }
     ,millstone: {
+        // Needs to be writable by server user
         cache_basedir: '/home/ubuntu/tile_assets/'
     }
     ,redis: {

config/environments/staging.js.example

@@ -30,6 +30,7 @@ var config = {
       bufferSize: 64
     }
     ,millstone: {
+        // Needs to be writable by server user
         cache_basedir: '/home/ubuntu/tile_assets/'
     }
     ,redis: {

config/environments/test.js.example

@@ -30,6 +30,7 @@ var config = {
       bufferSize: 64
     }
     ,millstone: {
+        // Needs to be writable by server user
         cache_basedir: '/tmp/cdb-tiler-test/millstone'
     }
     ,redis: {