update macOS instructions (phalcon-orphanage#1715)

* update macOS instructions

macOS users should use `/usr/local/bin` instead of `/usr/bin` because of **SIP** or "System Integrity Protection" introduced in El Capitan update, where even root users are not allowed to edit anything in `/usr` `/bin` `/sbin` `/System` folders with exception being mentioned `/usr/local/bin`

* Update macOS instructions

Update instructions to be clear on which commands to use for different macOS versions

en/devtools-installation.md

@@ -87,7 +87,14 @@ cd phalcon-devtools/
 
 ![](/images/content/devtools-mac-2.png)
 
-Create a symbolic link to the phalcon.php script:
+Next, we'll create a symbolic link to the `phalcon.php` script. On El Capitan and newer versions of macOS:
+
+```bash
+ln -s ~/phalcon-devtools/phalcon.php /usr/local/bin/phalcon
+chmod ugo+x /usr/local/bin/phalcon
+```
+
+if you are running an older version:
 
 ```bash
 ln -s ~/phalcon-devtools/phalcon.php /usr/bin/phalcon