update instructions on customizing PurpleMine

update .gitignore file to file structure introduced in 581ea78

.gitignore

@@ -1,4 +1,4 @@
 /.sass-cache
 /node_modules
-/sass/_custom-variables.scss
+/src/sass/_custom-variables.scss
 /.sublime-grunt.cache

README.md

@@ -48,7 +48,11 @@ Now all the dependencies should be ready to use. Run one more command:
 
 And now the grunt is watching for changes in files placed in `src/` folder. Just change what you need, and it'll run SASS preprocessor automatically.
 
-Regrettably, optional file include is not possible in SASS, so I would recommend creating a new file, e.g. `_my_variables.scss` and importing it a the beginning of the `application.scss` file. That way all the variables with the `!default` flag could be overridden.
+Regrettably, optional file include is not possible in SASS, so I would recommend creating a new file, e.g. `src/sass/_custom-variables.scss` and importing it a the beginning of the `application.scss` file. That way all the variables with the `!default` flag could be overridden.
+
+The path `src/sass/_custom-variables.scss` is added to `.gitignore` so it should make upgrading PurpleMine with keeping your changes rather painless, given that the only thing you changed in PurpleMine's source was adding this one line with `@import "custom-variables";`.
+
+If you need to customize styles for [Redmine Backlogs][redmine_backlogs] remember to include your `_custom-variables.scss` in `src/sass/plugins/redmine_backlogs/_common.scss`.
 
 ## Changelog