pw_env_setup: Add README to environment directory

Change-Id: I77f4036d85c9a15e5a60bc0aafbed2020fe8b466
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/80580
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_env_setup/destination.md b/pw_env_setup/destination.md
new file mode 100644
index 0000000..37a1b9a
--- /dev/null
+++ b/pw_env_setup/destination.md
@@ -0,0 +1,9 @@
+# Pigweed Environment Directory
+
+This is the directory into which Pigweed installed its environment. In general,
+it's acceptable to delete this entire directory, but don't delete any files
+within this directory without deleting the entire directory or there may be
+strange results.
+
+After deleting this directory, run `. bootstrap.sh` or `bootstrap.bat` instead
+of `. activate.sh` or `activate.bat`.
diff --git a/pw_env_setup/util.sh b/pw_env_setup/util.sh
index 42c002a..d43c050 100644
--- a/pw_env_setup/util.sh
+++ b/pw_env_setup/util.sh
@@ -244,6 +244,8 @@
     "$_PW_PYTHON" "$PW_ROOT/pw_env_setup/py/pw_env_setup/env_setup.py" "$@"
     _PW_ENV_SETUP_STATUS="$?"
   fi
+
+  cp "$PW_ROOT/pw_env_setup/destination.md" "$_PW_ACTUAL_ENVIRONMENT_ROOT/README.md"
 }
 
 pw_activate() {