Add help blurb in README

README.md

@@ -2,4 +2,82 @@
 
 [![Build and test](https://github.com/dark/skyscraper-puzzle/actions/workflows/cmake-single-platform.yml/badge.svg)](https://github.com/dark/skyscraper-puzzle/actions/workflows/cmake-single-platform.yml)
 
-TODO text
+This is a toy project to create a generator and solver of **skyscraper
+puzzles**.
+
+```
+Usage: ./skyscraper (-m|--mode) mode [-z|--size size] [-s|--seed seed] [-o|--output-file filename] [-f|--solution-file filename] 
+Where:
+  --mode is the program execution mode (only 'create' is supported now)
+  --size is the board size (default: 5)
+  --seed is the seed to use for random creation (default: a random seed is used)
+  --output-file is the file where the puzzle should be printed (default: stdout)
+  --solution-file is the file where the solution should be printed (default: not printed)
+```
+
+## Puzzle rules and objectives
+
+A skyscraper puzzle is generated around a NxN board of numbers. Each
+number on the board is between 1 and N, and each row and column of the
+board contains each number (1 through N) exactly once. For example:
+
+```
+4 2 5 1 3
+3 1 4 5 2
+1 4 2 3 5
+5 3 1 2 4
+2 5 3 4 1
+```
+
+The idea is that the board is a neighborhood, and each square
+represents a skyscraper of a certain height (1 through N).
+
+The puzzle does not provide the content of the board, as the goal is
+to find the numbers in each square. Instead, the puzzle provides
+additional numbers around it. Those numbers represent how many
+"skyscrapers" are visible from each end of each row and column. Higher
+skyscrapers hide lower skyscrapers that are behind it.
+
+For example, take this row:
+
+```
+4 2 5 1 3
+```
+
+An observer from the left would "see" only two skyscrapers, i.e. the
+skyscraper with height `4` (in first position) and skyscraper with
+height `5` (in third position). This is because the skyscraper of
+height `2` (in second position) is hidden by the higher one in
+front. Similarly, those in fourth and fifth position are hidden.
+
+Similarly, an observer from the right would see only two skyscrapers,
+i.e. the skyscraper with height `5` (in third position) and skyscraper
+with height `3` (in fifth position).
+
+If we repeat this operation for each row and column, we obtain a set
+of values that we write around the border of the board, like this:
+
+
+```
+    2 3 1 2 2
+  -------------
+2 | 4 2 5 1 3 | 2
+3 | 3 1 4 5 2 | 2
+3 | 1 4 2 3 5 | 1
+1 | 5 3 1 2 4 | 2
+2 | 2 5 3 4 1 | 3
+  -------------
+    2 1 3 2 3
+```
+
+Or, if we remove the contents of the board:
+
+```
+ 2 3 1 2 2
+2         2
+3         2
+3         1
+1         2
+2         3
+ 2 1 3 2 3
+```