Added bootstrap 3 folder

bootstrap3/less/mixins/tab-focus.less

@@ -0,0 +1,9 @@
+// WebKit-style focus
+
+.tab-focus() {
+  // WebKit-specific. Other browsers will keep their default outline style.
+  // (Initially tried to also force default via `outline: initial`,
+  // but that seems to erroneously remove the outline in Firefox altogether.)
+  outline: 5px auto -webkit-focus-ring-color;
+  outline-offset: -2px;
+}