add environment variable to webtry local run documentation

experimental/webtry/README.md

 WebTry Server
 =============
 
 Allows trying out Skia code in the browser. Run a local webserver
 and from the pages it serves try out Skia code and see the results
 immediately. To make sandboxing easier this must be built w/GPU off.
 
 
 Running Locally
 ===============
 
-Set your SKIA_ROOT environment variable to point at your skia tree, then:
+One time setup:
+
+    $ export SKIA_ROOT=path_to_your_skia_source
+    $ export WEBTRY_INOUT=path_to_a_writeable_directory
+    $ mkdir -p $WEBTRY_INOUT
+
+Then, to run:
 
     $ cd experimental/webtry
     $ go get -d
     $ ./build
     $ ./webtry
 
 Then visit http://localhost:8000 in your browser.
 
 Only tested under linux and MacOS, doubtful it will work on other platforms.
 
@@ skipping 60 matching lines @@
 1. SSH into the server as default.
 2. cd ~/skia/experimental/webtry/setup
 3. git pull
 4. ./webtry_setup.sh
 
 Third Party Code
 ================
 
   * res/js/polyfill.js - Various JS polyfill libraries. To rebuild or update
     see poly/README.md.