README: Added explanation + example for config file

Author: erezsh

README.md

@@ -201,6 +201,57 @@ Options:
   - `--bisection-factor` - Segments per iteration. When set to 2, it performs binary search.
   - `--bisection-threshold` - Minimal bisection threshold. i.e. maximum size of pages to diff locally.
   - `-j` or `--threads` - Number of worker threads to use per database. Default=1.
+  - `--conf`, `--run` - Specify the run and configuration from a TOML file. (see below)
+
+### How to use with a configuration file
+
+Data-diff lets you load the configuration for a run from a TOML file.
+
+Reasons to use a configuration file:
+
+- Convenience - Set-up the parameters for diffs that need to run often
+
+- Easier and more readible - You can define the database connection settings as config values, instead of in a URI.
+
+- Gives you fine-grained control over the settings switches, without requiring any Python code.
+
+Use `--conf` to specify that path to the configuration file. data-diff will load the settings from `run.default`, if it's defined.
+
+Then you can, optionally, use `--run` to choose to load the settings of a specific run, and override the settings `run.default`. (all runs extend `run.default`, like inheritance).
+
+Finally, CLI switches have the final say, and will override the settings defined by the configuration file, and the current run.
+
+Example TOML file:
+
+```toml
+# Specify the connection params to the test database.
+[database.test_postgresql]
+driver = "postgresql"
+user = "postgres"
+password = "Password1"
+
+# Specify the default run params
+[run.default]
+update_column = "timestamp"
+verbose = true
+
+# Specify params for a run 'test_diff'.
+[run.test_diff]
+verbose = false
+# Source 1 ("left")
+1.database = "test_postgresql"                      # Use options from database.test_postgresql
+1.table = "rating"
+# Source 2 ("right")
+2.database = "postgresql://postgres:Password1@/"    # Use URI like in the CLI
+2.table = "rating_del1"
+```
+
+In this example, running `data-diff --conf myconfig.toml --run test_diff` will compare between `rating` and `rating_del1`.
+It will use the `timestamp` column as the update column, as specified in `run.default`. However, it won't be verbose, since that
+flag is overwritten to `false`.
+
+Running it with `data-diff --conf myconfig.toml --run test_diff -v` will set verbose back to `true`.
+
 
 ## How to use from Python