Add git pull to README.md (#1460)

Minor addition to bring README.md in line with the web documentation.
Resolves #1447.

README.md

diff --git a/README.md b/README.md
index 4369f14..927f83e 100644
--- a/README.md
+++ b/README.md
@@ -20,6 +20,7 @@
 compiling from source). Basically, that amounts to
 
 ```
+git pull
 ./emsdk install latest
 ./emsdk activate latest
 ```