Add quick introduction in README.

The goal is to provide minimal information required for new users to
start using the command:
1. an exact command to install the package (I am new to python and I did
   not know how to do this)
1. simple command to fetch one stock, to ensure installation was
   successful
1. information about required `price` metadata in beancount file
1. infomration about --update flag to fetch most recent prices

I also added the command for the type checker that is used by the CI.

Refs beancount#33

README.md

@@ -9,21 +9,46 @@ This used to be located within Beancount itself (at v2) under beancount.prices.
 This repo will contain all future updates to that script and to those price
 sources.
 
-
 ## Documentation
 
 Some documentation is still part of Beancount. More about how to use this can be
 found on that [mailing-list](https://groups.google.com/forum/#!forum/beancount).
 Otherwise read the source.
 
+## Quick start
+
+To install beanprice, run:
+
+```shell
+pip install git+https://github.com/beancount/beanprice.git
+```
+
+You can fetch the latest price of a stock by running:
+
+```shell
+bean-price -e 'USD:yahoo/AAPL'
+```
+
+To fetch the latest prices from your beancount file, first ensure that commodities have price metadata, e.g.
+
+```
+2000-01-01 commodity AAPL
+  price: "USD:yahoo/AAPL"
+```
 
-## Download & Installation
+Then run:
 
-You can obtain the source code from the official Git repository on Github:
+```shell
+bean-price ledger.beancount
+```
 
-  | https://github.com/beancount/beanprice/
+To update prices up to the present day, run:
+
+```shell
+bean-price --update ledger.beancount
+```
 
-Install using the usual pip incantation.
+For more detailed guide for price fetching, read <https://beancount.github.io/docs/fetching_prices_in_beancount.html>.
 
 ## Testing
 
@@ -39,6 +64,12 @@ Lint:
 pylint beanprice
 ```
 
+Type checker:
+
+```
+mypy beanprice --ignore-missing-imports
+```
+
 ## Copyright and License
 
 Copyright (C) 2007-2020  Martin Blais.  All Rights Reserved.