add a page on runtime environments

RuntimeEnvironments.md

@@ -0,0 +1,11 @@
+# Runtime Environments
+
+This page contains notes about using various runtime environments inside Isolate 
+sandboxes.
+
+## Python
+
+Due to a [Python bug](http://bugs.python.org/issue10496), the interpreter 
+requires the `$HOME` variable to be set to something. Otherwise, it tries to 
+look up the UID used by Isolate in `/etc/passwd` and fails. Setting `HOME=/box` 
+seems to work.