Update README.markdown (#425)

Adding environment variable clarification to readme

Author: emchap

README.markdown

@@ -80,7 +80,16 @@ Run <code><strong>src login <i>SOURCEGRAPH-URL</i></strong></code> to authentica
 - `SRC_ENDPOINT`: the URL to your Sourcegraph instance (such as `https://sourcegraph.example.com`)
 - `SRC_ACCESS_TOKEN`: your Sourcegraph access token (on your Sourcegraph instance, click your user menu in the top right, then select **Settings > Access tokens** to create one)
 
-For convenience, you can export these environment variables in your shell profile. You can also inline them in a single command with:
+For convenience, you can export these environment variables in your shell profile. To configure them as environment variables, add the following to your terminal profile file, typically accessible at `~/.bash_profile` (if using Bash) or `~/.zprofile` (if using Zsh):
+
+```
+export SRC_ACCESS_TOKEN=my-token
+export SRC_ENDPOINT=https://sourcegraph.example.com 
+```
+
+Replace `my-token` and `https://sourcegraph.example.com` with the appropriate values for your account and instance.
+
+You can also inline them in a single command with:
 
 ```sh
 SRC_ENDPOINT=https://sourcegraph.example.com SRC_ACCESS_TOKEN=my-token src search 'foo'