update readme to support proxy (#131)

README.md

@@ -15,6 +15,20 @@ You can get the tenant Id from Azure portal or the Azure CLI. If you need to fet
 
 You can now use any of the authentication methods and pass in the tenant Id or use the `setDomain()` method on the existing credential to change the tenant it uses to create the tokens.
 
+### Known issue with proxy support
+
+This package uses `adal-node` which has moved to `axios` from `request` as the package of choice for httpRequests since npm support for `request` is discontinued. However it was noticed later that [`axios` does not support proxies](https://github.com/axios/axios/issues/2072). The `adal-node` package is taking only security changes and will not be fixing this issue. Therefore, we recommend using `https-proxy-agent` as suggested in the issue linked in case you need proxies for your application. For example: 
+
+```
+// paste this code to support proxy environment
+// reference:
+// https://github.com/axios/axios/issues/2072#issuecomment-567473812
+const HttpsProxyAgent = require('https-proxy-agent');
+const axios = require('axios')
+axios.defaults.proxy = false
+axios.defaults.httpsAgent = new HttpsProxyAgent('http://MY_PROXY:8888')
+```
+
 ### Example
 
 ### username/password based login