Doc: update versioning policy

Enable "pre-release identifiers" and add information on hierarchical
versioning scheme.

Change-Id: If1bd29d2e3d8a97cbcbefd504fbf4f042f695fd8
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
diff --git a/docs/deployments/libraries.rst b/docs/deployments/libraries.rst
index 2e60d86..25902ef 100644
--- a/docs/deployments/libraries.rst
+++ b/docs/deployments/libraries.rst
@@ -4,6 +4,8 @@
 other deployments or external applications. The following library deployments
 are currently supported:
 
+.. _libs-libts:
+
 libts
 -----
 Userspace applications that depend on trusted services may use *libts* for handling
@@ -29,6 +31,8 @@
   * - Used by
     - * Userspace applications
 
+.. _libs-libsp:
+
 libsp
 -----
 *libsp* provides a functional interface for using FF-A messaging and memory