Add README

Author: lannstark

README.md

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+# Excel Module
+
+General Excel Module to implement Excel Download simply and fast.
+
+## Usage - Spring Boot with Rest API 
+
+Excel Module uses annotation and reflections to render excel file.
+
+You need to use @ExcelColumn to Object to be rendered in excel file.
+
+See below example.
+
+### Jitpack
+
+Here, only explain how to manage dependency with gradle.
+
+See other build tool usage from [Jitpack Hompage](https://jitpack.io/) 
+
+```
+allprojects {
+    repositories {
+        maven { url 'https://jitpack.io' }
+    }
+}
+
+// Use proper version
+dependencies {
+  implementation('com.github.lannstark:excel-download:0.1.1')
+}
+```
+
+### Server
+
+```java
+// In ExcelDeto
+// Rendered Field Order is same as Dto field order
+public class ExcelDto {
+
+  // Annotation Case 1. no headerStyle and contentsStyle
+  @ExcelColumn(headerName = "User Name")
+  private String name;
+  
+  // Annotation Case 2. use not default style, but use defined style
+  @ExcelColumn(headerName = "User Age",
+      headerStyle = @ExcelColumnStyle(excelCellStyleClass = DefaultExcelCellStyle.class, enumName = "BLUE_HEADER")
+  )
+  private int age;
+  
+  // Annotation Case 3. You can also configure contents style
+  @ExcelColumn(headerName = "Happy BirthDay",
+      contentsStyle = @ExcelColumnStyle(excelCellStyleClass = DefaultExcelCellStyle.class, enumName = "CONTENTS")
+  )
+  private LocalDate birthDay;
+
+}
+```
+
+If you want to config default style in class, you should use @DefaultHeaderStyle or @DefaultContentsStyle.
+This style will be applied to all fields having not field style in this class.
+
+```java
+@DefaultHeaderStyle(
+    style = @ExcelColumnStyle(excelCellStyleClass = DefaultExcelCellStyle.class, enumName = "BLUE_HEADER")
+)
+@DefaultContentsStyle(
+	style = @ExcelColumnStyle(excelCellStyleClass = DefaultExcelCellStyle.class, enumName = "CONTENTS")
+)
+public class ExcelDto {
+	
+	private String name;
+    
+    private int age;
+    
+    private LocalDate birthDay;
+	
+}
+```
+
+Additionally, There is customizing use case, not pre-defined excel cell style in DefaultExcelCellStyle.
+
+See Customizing section
+
+
+```java
+// In Controller
+@RestController
+public class Controller {
+	
+  @GetMapping("/any-url")
+  public void methodName(RequestDto requestDto, HttpServletResponse response) throws IOException {
+    // If you specify response type when you use axios,
+    // you don't need to set HttpServletResponse contenttype. See #1 in Front with axios section
+    response.setContentType("application/vnd.ms-excel");
+  
+    List<ExcelDto> excelDtos = someService.getRenderedData(requestDto);
+    ExcelFile excelFile = new OneSheetExcelFile<>(excelDtos, ExcelDto.class);
+    excelFile.write(response.getOutputStream());
+  }
+
+}
+```
+
+### Front with axios
+```js
+axios({
+  method: 'GET',
+  url: 'server-url',
+  responseType: 'blob' // MUST NEED
+})
+.then(response => {
+    // #1 Here, I designate type with response header, however, you can specify 'application/vnd.ms-excel'
+    const url
+      = window.URL.createObjectURL(new Blobk([response.data], {type : response.headers['content-type']}));
+    const link = document.createElement('a');
+    link.href = url;
+    link.setAttribute('download', 'ExcelFile.xlsx');
+    document.body.appendChild(link);
+    link.click();
+});
+```
+
+
+## Customizing
+
+You can create custom ExcelCellStyle with Class or Enum
+
+If you use Class, you don't need to designate enumName of @ExcelColumnStyle
+```java
+public class BlueHeaderStyle implements ExcelCellStyle {
+
+  @Override
+  public void apply(CellStyle cellStyle) {
+    // Do anything you want to change style
+    cellStyle.setBlaBla();
+  }
+
+} 
+``` 
+
+For convenient custom style class, we provide template style class, CustomExcelCellStyle
+You can set
+- cell color
+- 4-side borders type
+- cell contents align
+Other features will be updated gradually.  
+
+```java
+public class BlueHeaderStyle extends CustomExcelCellStyle {
+
+	@Override
+	public void configure(ExcelCellStyleConfigurer configurer) {
+		configurer.foregroundColor(223, 235, 246)
+				.excelBorders(DefaultExcelBorders.newInstance(ExcelBorderStyle.THIN))
+				.excelAlign(DefaultExcelAlign.CENTER_CENTER);
+	}
+
+}
+```
+
+```java
+@DefaultHeaderStyle(
+    style = @ExcelColumnStyle(excelCellStyleClass = BlueHeaderStyle.class)
+)
+public class ExcelDto {
+
+  private String field1;
+
+}
+```
+
+
+If you use Enum, you have to specify enumName of @ExcelColumnStyle
+```java
+public enum CustomCellStyle implements ExcelCellStyle {
+	
+  CUSTOM_HEADER(field1, field2);
+  
+  // Whatever fields you need to configure cell style
+  private final String field1;
+  private final int field2;
+  
+  @Override
+  public void apply(CellStyle cellStyle) {
+    // Do anything you want to change style with defined enum fields.
+    cellStyle.setBlaBla();
+  }
+  
+}
+```  
+
+```java
+public class ExcelDto {
+
+  @ExcelColumn(headerName = "Field Header Title",
+      contentsStyle = @ExcelColumnStyle(excelCellStyleClass = CustomCellStyle.class, enumName = "CUSTOM_HEADER")
+  )
+  private String field1;
+
+}
+```
+
+## Kinds of Excel File
+
+- OneSheetExcelFile
+- MultiSheetExcelFile