Add Badge for LGPLv2.1+ License

As we are now hosting our own copy of ROOT, a badge for its license is
also needed.

badges/license_lgpl2.1+.svg

+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" width="114" height="20">
+    <linearGradient id="b" x2="0" y2="100%">
+        <stop offset="0" stop-color="#bbb" stop-opacity=".1"/>
+        <stop offset="1" stop-opacity=".1"/>
+    </linearGradient>
+    <mask id="a">
+        <rect width="114" height="20" rx="3" fill="#fff"/>
+    </mask>
+    <g mask="url(#a)">
+        <path fill="#555" d="M0 0h45v20H0z"/>
+        <path fill="#1081c1" d="M45 0h69v20H45z"/>
+        <path fill="url(#b)" d="M0 0h114v20H0z"/>
+    </g>
+    <g fill="#fff" text-anchor="middle" font-family="DejaVu Sans" font-size="11">
+        <text x="23" y="15" fill="#010101" fill-opacity=".3">license</text>
+        <text x="22" y="14">license</text>
+    </g>
+    <g fill="#fff" text-anchor="middle" font-family="DejaVu Sans" font-size="11">
+        <text x="80" y="15" fill="#010101" fill-opacity=".3">LGPLv2.1+</text>
+        <text x="79" y="14">LGPLv2.1+</text>
+    </g>
+</svg>
\ No newline at end of file