contributing: fix monero qr code and add alt text

_i18n/en.yml

@@ -421,6 +421,8 @@ contributing:
   sponsor_p: You can also support Monero through a sponsorship program. See the dedicated 
   sponsor_link: Sponsorships page
   sponsor_p2: for more info.
+  altqrmonero: Monero QR code
+  altqrbitcoin: Bitcoin QR code
 
 faq:
   intro: Find answers to recurring questions and concerns about Monero.

css/custom.css

@@ -4280,6 +4280,10 @@ p.hangouts-social {
     margin: 0;
 }
 
+.contribute img.qr {
+    max-width: 15rem;
+}
+
 /**************************MONERO PROJECT***********************/
 
 .monero-project p {

get-started/contributing/index.md

@@ -76,10 +76,10 @@ permalink: /get-started/contributing/index.html
                     </div><br>
                     <div class="row center-xs">
                         <div class="col-lg-6">
-                            <img src="/img/donate-monero.png" alt=""/>
+                            <img class="qr" src="/img/donate-monero.png" alt="{% t contributing.altqrmonero %}"/>
                         </div>
                         <div class="col-lg-6">
-                            <img src="/img/donate-bitcoin.png" alt=""/>
+                            <img class="qr" src="/img/donate-bitcoin.png" alt="{% t contributing.altqrbitcoin %}"/>
                         </div>
                     </div>
                     <div class="row start-xs">