add lib-icu and hhvm

doc/02-libraries.md

@@ -33,8 +33,11 @@ installed on the system but are not actually installable by Composer. This
 includes PHP itself, PHP extensions and some system libraries.
 
 * `php` represents the PHP version of the user, allowing you to apply
-   constraints, e.g. `>=5.4.0`. To require a 64bit version of php, you can
-   require the `php-64bit` package.
+  constraints, e.g. `>=5.4.0`. To require a 64bit version of php, you can
+  require the `php-64bit` package.
+
+* `hhvm` represents the version of the HHVM runtime (aka HipHop Virtual
+  Machine) and allows you to apply a constraint, e.g., '>=2.3.3'.
 
 * `ext-<name>` allows you to require PHP extensions (includes core
   extensions). Versioning can be quite inconsistent here, so it's often
@@ -42,8 +45,8 @@ includes PHP itself, PHP extensions and some system libraries.
   package name is `ext-gd`.
 
 * `lib-<name>` allows constraints to be made on versions of libraries used by
-  PHP. The following are available: `curl`, `iconv`, `libxml`, `openssl`,
-  `pcre`, `uuid`, `xsl`.
+  PHP. The following are available: `curl`, `iconv`, `icu`, `libxml`,
+  `openssl`, `pcre`, `uuid`, `xsl`.
 
 You can use `composer show --platform` to get a list of your locally available
 platform packages.