[docs] Link semantic versioning to semver.org

doc/01-basic-usage.md

@@ -77,11 +77,12 @@ Version constraints can be specified in a few different ways.
 * **Next Significant Release (Tilde Operator):** The `~` operator is best
   explained by example: `~1.2` is equivalent to `>=1.2,<2.0`, while `~1.2.3` is
   equivalent to `>=1.2.3,<1.3`. As you can see it is mostly useful for projects
-  respecting semantic versioning. A common usage would be to mark the minimum
-  minor version you depend on, like `~1.2` (which allows anything up to, but not
-  including, 2.0). Since in theory there should be no backwards compatibility
-  breaks until 2.0, that works well. Another way of looking at it is that using
-  `~` specifies a minimum version, but allows the last digit specified to go up.
+  respecting [semantic versioning](http://semver.org/). A common usage would be
+  to mark the minimum minor version you depend on, like `~1.2` (which allows
+  anything up to, but not including, 2.0). Since in theory there should be no
+  backwards compatibility breaks until 2.0, that works well. Another way of
+  looking at it is that using `~` specifies a minimum version, but allows the
+  last digit specified to go up.
 
 By default only stable releases are taken into consideration. If you would like
 to also get RC, beta, alpha or dev versions of your dependencies you can do