sponsors: acknowledge and credit GitHub and IBB as top sponsors

Closes #397

_index.html

@@ -92,6 +92,8 @@
   <img class="logo" src="pix/haxx.svg" alt="Haxx">
   <img class="logo" src="pix/wolfssl.svg" alt="wolfSSL">
   <img class="logo" src="pix/fastly.svg" alt="Fastly">
+  <img class="logo" src="pix/top/github.svg" alt="GitHub">
+  <img class="logo" src="pix/top/ibb.svg" alt="IBB">
   <img class="logo" src="pix/teamviewer.svg" alt="Teamviewer">
   </a>
 

_sponsors.html

@@ -60,19 +60,29 @@
 <p>
   <a class="x" href="https://www.haxx.se/" rel="sponsored"><img src="pix/haxx.svg" width="500" height="169" border="0" alt="Haxx"></a>
 <p>
- Haxx is the primary sponsor of the curl project.
+ Haxx is the primary sponsor of the curl project
 
 <p>
 <a class="x" href="https://www.wolfssl.com/" rel="sponsored"><img src="pix/wolfssl.svg" width="500" height="387" alt="wolfSSL"></a>
 <p>
  wolfSSL employs <a href="https://daniel.haxx.se/">Daniel</a> and lets him
- spend paid work hours on curl.
+ spend paid work hours on curl
 
 <p>
  <a class="x" href="https://www.fastly.com/" rel="sponsored"><img src="pix/fastly.svg" width="500" alt="Fastly"></a>
 <p>
- Fastly delivers the curl website and web contents to the world.
+ Fastly delivers the curl website and web contents to the world
 
+<p>
+ <a class="x" href="https://www.github.com/" rel="sponsored"><img src="pix/top/github.svg" width="500" alt="GitHub"></a>
+<p>
+ GitHub sponsors CI infrastructure
+
+<p>
+ <a class="x" href="https://www.hackerone.com/internet-bug-bounty" rel="sponsored"><img src="pix/top/ibb.svg" width="500" alt="IBB"></a>
+<p>
+ The Internet Bug Bounty sponsors the curl bug bounty
+
 <p>
  <a class="x" href="https://www.teamviewer.com/" rel="sponsored"><img src="pix/teamviewer.svg" width="700" alt="Teamviewer"></a>
 <p>