Fix documentation: the --force is required

Without the `--force` option we get an error:

```
Failed to download pear/php_archive within preferred state "stable", latest release is version 0.12.0, stability "alpha", use "channel://pear.php.net/php_archive-0.12.0" to install
install failed
```

Author: mnapoli

README.CONTRIBUTING

@@ -120,7 +120,7 @@ mkdir go-pear-tarballs
 cd go-pear-tarballs
 pear download -Z PEAR-stable Archive_Tar-stable Console_Getopt-stable XML_Util-stable
 cd ../..
-pear install PHP_Archive
+pear install --force PHP_Archive
 php make-installpear-nozlib-phar.php
 php make-gopear-phar.php
 
@@ -246,4 +246,4 @@ as if it were the remote one.  This can be used to debug complex server interact
 without requiring any internet access whatsoever.
 
 Again, as with all other tasks, consult one of the experienced developers if you need
-assistance setting up pearweb locally, or have any questions!
+assistance setting up pearweb locally, or have any questions!