Add info about PKG_CONFIG_PATH and LD_LIBRARY_PATH when using radare2 from /usr/local

[crazyquark]

Build on systems where radare2 is in /usr/local

Checklist

• [ ] Closing issues: #issue
• [x] Mark this if you consider it ready to merge
• [ ] I've added tests (optional)
• [x] I wrote some documentation

Description