Simple explanation of the program on README.

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@@ -5,13 +5,169 @@ implementation only covers a few commands of the original ledger-cli. This
 project is only for educational purposes. It does not provide any advantages
 over the original project.
 
-# What it has
-
-- Support for commands:
-    - [X] Register.
-    - [X] Balance.
-    - [X] Print.
-- Support for flags:
-    - [X] `--sort`.
-    - [ ] `--price-db`.
-    - [X] `--file`.
+# Usage
+
+You can use this program as if it was the original **ledger-cli**, but with a
+selected group of flags and commands:
+```
+./ledger.py [-h] [-v] [-S value-expression] [--price-db FILE] [-f FILE] Action [Action ...]
+```
+
+- `-h`: Displays the usage and description of the program.
+- `-v`: Displays the current version of the program.
+- `-S` or `--sort`: Sorts the entries. See the [`--sort` section](#sort) for
+  more details.
+- `--price-db`: Currently it has no purpose. See the [`--price-db`
+  section](#price-db) for more details.
+- `-f` or `--file`: Use the specified file as an input.
+- `Action`: The first word will be our *verb*. The *verb* describes what our
+  program will output. It currently can be `register`, `balance` or `print`.
+
+  The following words will be our *filters* for which accounts we'll work with.
+  See the [filters section](#filters) for more details.
+
+Let's see this commands in more detail.
+
+
+# Filters
+
+Filters can be applied to just see a subset of all of our accounts. To use
+filters, we'll start by describing our keywords:
+
+### And
+
+```
+{x} and {y}
+```
+
+This way, we can filter accounts if they have the text `{x}` and `{y}` in them.
+
+> Examples:
+> 
+> ```
+> Expenses and Amazon
+> ```
+> ```
+> Income and Job and Encora
+> ```
+
+### Or
+
+```
+{x} or {y}
+```
+```
+{x} {y}
+```
+
+Both ways work. We can filter accounts if they have either `{x}` or `{y}`.
+
+> Examples:
+> ```
+> Assets Liabilities
+> ```
+> ```
+> Assets and (Bank or Wallet)
+> ```
+
+### Not
+
+```
+not {x}
+```
+
+Filter accounts that DO NOT have `{x}` in them.
+
+> Examples:
+> ```
+> (Liabilities and not Rent) or Expenses
+> ```
+> ```
+> Expenses and not (Drinks or Food)
+> ```
+
+### Regex
+
+As you've just read. Every word is treated as a regex. For this project, we are
+using [Python's regex](https://docs.python.org/3/library/re.html). Feel free to
+experiment with this function. The only restriction is to not use spaces, as
+they are our limiters for keywords and such.
+
+> Examples:
+> ```
+> Income and [^:]+:[^:]+:[^:]+
+> ```
+> ```
+> ^Expenses and not .*tion$
+> ```
+
+~~Sorry for my poor examples. I don't know about finances. ;-;~~
+
+
+# `--file`
+
+This flag can be used multiple times. It indicates the ledger file(s) we will be
+working on. ~~If no file is specified, the program will read from standard input
+(`stdin`).~~ ***NOT YET IMPLEMENTED.***
+
+
+# `--sort`
+
+**The `--sort` flag will always sort in ascending order.** We can sort our
+entries in two different ways:
+
+- `date` or `d`: Sort every entry given the date of the transaction.
+- `amount` or `a`: Sort given the amount of the transaction. **This breaks
+  entries into individual transactions.**
+
+## It doesn't work on `balance`
+
+The `--sort` flag will only make changes on the outputs of `register` and
+`print`. The verb `balance` will **always** print in alphabetical order.
+
+
+# `--price-db`
+
+It points to the file that contains our prices history.
+
+## It doesn't work well alone
+
+The original flag for `--price-db` is commonly used alongside other flags such
+as `-V` or `--market`, that uses the prices history to convert all amounts into
+our default currency. That's why currently this command doesn't have any
+function to it. It's yet to know how we will work with this flag.
+
+
+# Examples
+
+```
+$ ./ledger.py -f index.ledger -S d register Bank  
+11-Nov-21 Payment for hard.. Bank:Paypal                  $350.00      $350.00
+12-Jul-01 Partial payment .. Bank:Paypal                  $100.00      $450.00
+12-Nov-16 Sold some bitcoins Bank:Paypal                   $42.21      $492.21
+12-Nov-29 Purchased bitcoins Bank:Paypal                 $-300.00      $192.21
+```
+
+```
+$ ./ledger.py -f index.ledger --sort a print .\*coin  # In ZSH, the `*` must be escaped.
+2013/02/20 Purchased reddit gold for the year
+    Asset:Bitcoin Wallet                  -10.00 BTC
+
+2012/11/16 Sold some bitcoins
+    Asset:Bitcoin Wallet                   -3.50 BTC
+
+2012/11/29 Purchased bitcoins
+    Asset:Bitcoin Wallet                   15.00 BTC
+```
+
+```
+$ ./ledger.py --file index.ledger bal Bank: Expense:
+             $192.21  Bank:Paypal
+           10.00 BTC
+              $10.00  Expense
+              $10.00    Favor
+           10.00 BTC    Web Services:Reddit
+--------------------
+           10.00 BTC
+             $202.21
+```