Add usage to readme

README.md

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 # gmail-send
 Command line interface for sending Gmail emails
+
+## Prerequisites
+
+* [Python 3](https://python.org)
+
+## Usage
+
+```
+usage: gmail-send.py [-h] [-s SUBJECT] [-v] [--html]
+                     gmail_user gmail_pwd recipient
+
+Send an email message via SMTP. Message body is read from stdin.
+
+positional arguments:
+  gmail_user            sender's gmail username (without domain)
+  gmail_pwd             sender's gmail password
+  recipient             recipient's email address (with domain)
+
+optional arguments:
+  -h, --help            show this help message and exit
+  -s SUBJECT, --subject SUBJECT
+                        email subject
+  -v, --verbose         output sent message
+  --html                send email as HTML
+```