update(README): add installation and running instructions

README.md

-# preacher-telegram-bot
+# PreacherBot
 Preacherbot is a community developed Telegram bot that facilitates the weekly Church of Monero Mass.
+
+## Installation
+`docker-compose build`
+
+## Running
+First, you will need to export the following environment variables:
+
+`PREACHER_TELEGRAM_TOKEN` - your Telegram bot's access token
+
+`MONGO_INITDB_ROOT_USERNAME` (optional, defaults to "root")
+
+`MONGO_INITDB_ROOT_PASSWORD` (optional, defaults to "default_password")
+
+
+Then start the container in the background:
+
+`docker-compose up -d`