Clarify env var substitution format (theskumar#221)

timhughes · Saurabh Kumar · commit c5b47f42f9b2 · 2020-01-16T07:14:25.000+05:30
diff --git a/README.md b/README.md
@@ -41,7 +41,9 @@ export SECRET_KEY=YOURSECRETKEYGOESHERE
 
 `.env` can interpolate variables using POSIX variable expansion,
 variables are replaced from the environment first or from other values
-in the `.env` file if the variable is not present in the environment.
+in the `.env` file if the variable is not present in the environment. 
+Ensure that variables are surrounded with `{}` like `${HOME}` as bare 
+variables such as `$HOME` are not expanded.
 (**Note**: Default Value Expansion is not supported as of yet, see
 [\#30](https://github.com/theskumar/python-dotenv/pull/30#issuecomment-244036604).)
 
