make install.sh tell us what it is installing and why

Author: nikomatsakis

ci/install.sh

@@ -2,9 +2,15 @@
 set -ex
 
 if command -v mdbook >/dev/null 2>&1; then
+    echo "installing mdbook"
     cargo install mdbook --vers "0.0.28"
+else
+    echo "mdbook already installed at $(which mdbook)"
 fi
 
 if command -v ghp-import >/dev/null 2>&1; then
+    echo "installing ghp-import"
     pip install ghp-import
+else 
+    echo "ghp-import already installed at $(which ghp-import)"
 fi