Update readme with doc links and better package description (#26)

* Update readme with doc links and better package description

* Update readme with doc links and better package description

* Update readme with doc links and better package description

* Update readme with doc links and better package description

* Update README.md

Co-authored-by: Alexey Kopytko <alexey@kopytko.com>

* Update README.md

Co-authored-by: Alexey Kopytko <alexey@kopytko.com>

* Update README.md

Co-authored-by: Alexey Kopytko <alexey@kopytko.com>

* Update README.md

Co-authored-by: Alexey Kopytko <alexey@kopytko.com>

* Update README.md

Co-authored-by: Alexey Kopytko <alexey@kopytko.com>

README.md

@@ -2,12 +2,19 @@
 [![Latest Stable Version](https://poser.pugx.org/pear/spreadsheet_excel_writer/v/stable)](https://packagist.org/packages/pear/spreadsheet_excel_writer)
 [![Coverage Status](https://coveralls.io/repos/github/pear/Spreadsheet_Excel_Writer/badge.svg?branch=master)](https://coveralls.io/github/pear/Spreadsheet_Excel_Writer?branch=master)
 
+# Spreadsheet_Excel_Writer
+
 This package is [Spreadsheet_Excel_Writer](http://pear.php.net/package/Spreadsheet_Excel_Writer) and has been migrated from [svn.php.net](https://svn.php.net/repository/pear/packages/Spreadsheet_Excel_Writer).
 
 Please report all new issues [via the PEAR bug tracker](http://pear.php.net/bugs/search.php?cmd=display&package_name[]=Spreadsheet_Excel_Writer&order_by=ts1&direction=DESC&status=Open).
 
 If this package is marked as unmaintained and you have fixes, please submit your pull requests and start discussion on the pear-qa mailing list.
 
+
+# Installation
+
+## Pear
+
 To test, run
 
     $ phpunit
@@ -35,3 +42,124 @@ To install from Composer
 To install the latest development version
 
     $ composer require pear/spreadsheet_excel_writer:dev-master
+
+# Features
+
+- writing Excel (.XLS) spreadsheets
+- support: strings (with formatting for text and cells), formulas, images (BMP) 
+
+# Limitations
+Library support only 2 types of format for writing XLS, also known as Binary Interchange File Format ([BIFF](https://www.openoffice.org/sc/excelfileformat.pdf)): 
+- BIFF5 (Excel 5.0 - Excel 95)
+- BIFF8 (Excel 98 - Excel 2003)
+
+**Some important limitations:**  
+
+| Limit | BIFF5 | BIFF8 |
+| --- | --- | --- |
+| Maximum number of rows | 16384 | 65535 |
+| Maximum number of columns | 255 | 255 |
+| Maximum data size of a record | 2080 bytes | 8224 bytes |
+| Unicode support | CodePage based character encoding | UTF-16LE |
+
+Explanation of formats and specifications you can find [here](https://www.loc.gov/preservation/digital/formats/fdd/fdd000510.shtml) (section "Useful references")
+
+
+# Usage
+
+## Basic usage
+```php
+use Spreadsheet_Excel_Writer;
+
+
+$filePath = __DIR__ . '/output/out.xls';
+$xls = new Spreadsheet_Excel_Writer($filePath);
+
+// 8 = BIFF8
+$xls->setVersion(8);
+
+$sheet = $xls->addWorksheet('info');
+
+// only available with BIFF8
+$sheet->setInputEncoding('UTF-8');
+
+$headers = [
+    'id',
+    'name',
+    'email',
+    'code',
+    'address'
+];
+
+$row = $col = 0;
+foreach ($headers as $header) {
+    $sheet->write($row, $col, $header);
+    $col++;
+}
+
+for ($id = 1; $id < $max; $id++) {
+    $data = [
+        'id' => $id,
+        'name' => 'Name Surname',
+        'email' => 'mail@gmail.com',
+        'password' => 'cfcd208495d565ef66e7dff9f98764da',
+        'address' => '00000 North Tantau Avenue. Cupertino, CA 12345. (000) 1234567'
+    ];
+    $sheet->writeRow($id, 0, $data);
+}
+
+$xls->close();
+```
+
+## Format usage
+```php
+$xls = new Spreadsheet_Excel_Writer();
+
+$titleFormat = $xls->addFormat(); 
+$titleFormat->setFontFamily('Helvetica');
+$titleFormat->setBold();
+$titleFormat->setSize(10);
+$titleFormat->setColor('orange'); 
+$titleFormat->setBorder(1);
+$titleFormat->setBottom(2);
+$titleFormat->setBottomColor(44);
+$titleFormat->setAlign('center');
+
+$sheet = $xls->addWorksheet('info'); 
+
+$sheet->write(0, 0, 'Text 123', $titleFormat);
+```
+
+## Header usage (Sending HTTP header for download dialog)
+```php
+$xls = new Spreadsheet_Excel_Writer();
+$xls->send('excel_'.date("Y-m-d__H:i:s").'.xls');
+```
+
+
+# Performance
+
+**Platform:**  
+Intel(R) Core(TM) i5-4670 CPU @ 3.40GHz  
+PHP 7.4    
+
+**Test case:**  
+Write xls (BIFF8 format, UTF-8), by 5 cells (1x number, 4x string without format/styles, average line length = 120 char) in each row  
+
+**Estimated performance:**  
+
+| Number of rows | Time (seconds) | Peak memory usage (MB) |
+| --- | --- | --- |
+| 10000 | 0.2 | 4 |
+| 20000 | 0.4 | 4 |
+| 30000 | 0.6 | 6 |
+| 40000 | 0.8 | 6 |
+| 50000 | 1.0 | 8 |
+| 65534 | 1.2 | 8 |
+
+# Alternative solutions
+
+- [PHPOffice/PhpSpreadsheet](https://github.com/PHPOffice/PhpSpreadsheet)  
+File formats supported: https://phpspreadsheet.readthedocs.io/en/latest/  
+- [box/spout](https://github.com/box/spout)  
+File formats supported: https://opensource.box.com/spout/