Add thumbnail image when Twitter sharing (#199)

diff --git a/static/images/skywalking_400x400.png b/static/images/skywalking_400x400.png
new file mode 100644
index 0000000..5299759
--- /dev/null
+++ b/static/images/skywalking_400x400.png
Binary files differ
diff --git a/themes/docsy/layouts/partials/head.html b/themes/docsy/layouts/partials/head.html
index c731a68..527b504 100644
--- a/themes/docsy/layouts/partials/head.html
+++ b/themes/docsy/layouts/partials/head.html
@@ -1,5 +1,6 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
+<meta property="og:image" content="https://skywalking.apache.org/images/skywalking_400x400.png">
 {{ hugo.Generator }}
 <META NAME="ROBOTS" CONTENT="INDEX, FOLLOW">
 {{ range .AlternativeOutputFormats -}}