Check in SourceCred logo files (#636)

Summary:
The SVG was artisanally crafted by yours truly, and rasterized by the
accompanying script (which is fully deterministic, and also artisanal).

Test Plan:
Run `./src/assets/logo/rasterize.sh`, and note that the output is
unchanged.

wchargin-branch: add-logo

src/assets/logo/rasterize.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+set -eu
+
+main() {
+    cd "$(dirname "$0")"
+    for size in 1024 512 256 128 32 64 16; do
+        rasterize "${size}"
+    done
+}
+
+# rasterize SIZE
+rasterize() {
+    size=$1
+    inkscape -z ./sourcecred.svg -e "./sourcecred_${size}.png" \
+        -w "${size}" -h "${size}"
+}
+
+main "$@"

src/assets/logo/sourcecred.svg

@@ -0,0 +1,20 @@
+<svg
+    version="1.1"
+    baseProfile="full"
+    width="256" height="256"
+    xmlns="http://www.w3.org/2000/svg"
+>
+    <rect width="100%" height="100%" fill="teal" />
+    <circle
+        cx="128"
+        cy="128"
+        r="48"
+        fill="none"
+        stroke="white"
+        stroke-width="24"
+    />
+    <path d="M64 64 L96 96" stroke="white" stroke-width="24" />
+    <path d="M192 64 L160 96" stroke="white" stroke-width="24" />
+    <path d="M64 192 L96 160" stroke="white" stroke-width="24" />
+    <path d="M192 192 L160 160" stroke="white" stroke-width="24" />
+</svg>