README: add 1.0.21 zip file and drop PEAR support

i'll add 1.0.21 to the PEAR channel if there's any demand for it
but if there is any demand for it that'd be news to me

README.md

@@ -51,8 +51,7 @@ SSH-2, SFTP, X.509, an arbitrary-precision integer arithmetic library, Ed25519 /
 * PHP4 compatible
 * Composer compatible (PSR-0 autoloading)
 * Install using Composer: `composer require phpseclib/phpseclib:~1.0`
-* Install using PEAR: See [phpseclib PEAR Channel Documentation](http://phpseclib.sourceforge.net/pear.htm)
-* [Download 1.0.20 as ZIP](http://sourceforge.net/projects/phpseclib/files/phpseclib1.0.20.zip/download)
+* [Download 1.0.21 as ZIP](http://sourceforge.net/projects/phpseclib/files/phpseclib1.0.21.zip/download)
 
 ## Security contact information