trace: Update documentation

Revamp the documentation for the new features, including a description of
the new features and documentation for the trace command.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/.gitattributes b/.gitattributes
index 1879a2d..d5931f0 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -4,3 +4,4 @@
 *.bmp binary
 *.ttf binary
 *.gz binary
+*.png binary