Add README

Author: palfrey

README.md

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+wait-for-db
+===========
+[![Build Status](https://travis-ci.com/palfrey/wait-for-db.svg?branch=master)](https://travis-ci.com/palfrey/wait-for-db)
+
+Basically, [wait-for-it](https://github.com/vishnubob/wait-for-it) but for databases.
+
+Often ([especially within Dockerised configurations](https://docs.docker.com/compose/startup-order/)) there is the problem of "I need the database to be up before step X will work". Now, if all you need is "up" then generally wait-for-it and the database port is enough. If on the other hand you've got migration scripts or other such things you need to run on your database, up isn't enough.
+
+wait-for-db lets you wait for a particular database to be both up (in the sense of "will let you connect without errors") and optionally "returns at least one row to a particular query". It will also attempt to fail immediately for permanent failures (e.g. syntax errors)
+
+Usage
+-----
+1. Download static binary from https://github.com/palfrey/wait-for-db/releases into your Docker image
+2. `./wait-for-db <options>`
+
+Options
+-------
+* `-m/--mode`: `postgres` or `odbc`
+* `-c/--connection-string`: Mode-appropriate connection string. So `postgresql://<username>:<pasword>@<host>:<port>` or `Driver=<path to driver>;<various ODBC options>` depending on your driver
+* `-s/--sql-text`: SQL query to run once connected. It should return at least one row, or will be regarded as failing. Default: no query, just be regarded as succeeding the moment it connects.
+* `-p/--pause`: Pause between attempts for non-permanent failures. Default is 3 seconds
+* `-t/--timeout`: Time to wait before failing entirely. Default is wait forever.
+
+Database support
+----------------
+* Anything you've got an [ODBC](https://en.wikipedia.org/wiki/Open_Database_Connectivity) driver for (which should be most SQL databases)
+* Postgres