Document current behavior (#15)

* document current behavior

* Mention outer try/except to avoid double sentry

Author: fiendish

README.md

@@ -8,7 +8,12 @@ status 0 if events are able to be sent to Sentry.
 It reads its configuration from the environment variable `$SHELL_SENTRY_DSN`
 and, if such a variable is found, removes it from the environment before
 calling the wrapped program. If that environment variable is not present, shentry will look
-for `$SENTRY_DSN`. If neither of the environment variables are present or btoh
+for `$SENTRY_DSN` (and similarly remove it from the environment).
+If you need to use SENTRY_DSN inside your project code, make sure to set both.
+You may also in that case want to put a top-level try/except around your whole
+program to prevent uncaught exceptions from trigging both your in-process sentry sdk
+and also your extra-process shentry, since you very likely only want one or the other.
+If neither of the environment variables are present or both
 are empty, shentry will try to read a DSN from `/etc/shentry_dsn`. If no DSN
 can be found, the wrapped will have normal behavior (stdout/stderr will go
 to their normal file descriptors, exit code will be passed through, etc).