update imagemagick symlink across to latest version

90% review

**Description**
When try to setup the application locally, I am having trouble with install rMagick because ImageMagic was installed via HomeBrew. The solution in README is really helpful to resolve the issue. However, it doesn't give any explicit hint to let users know there may be a need to change the ImageMagic path if their version is different. Therefore I updated the information in README to make it clear. Hope that will help.

**What to change**
- [x] Updated README