More useful links for Libsodium installation

Also mentioning one needs to install the PHP extension too.

INSTALL.md

@@ -13,7 +13,7 @@ the options and adjust them as you see fit.
 - PHP version 5.3 or above
 - _one_ of the following sources of cryptographically safe randomness is required:
   - PHP 7 or higher
-  - [Libsodium](https://download.libsodium.org)
+  - [Libsodium](https://download.libsodium.org/libsodium/content/installation/) and it's [PHP extension](https://paragonie.com/book/pecl-libsodium/read/00-intro.md#installing-libsodium)
   - open_basedir access to `/dev/urandom`
   - mcrypt extension
   - com_dotnet extension