Docs: Updated readme

Author: roman.vasilev

README.md

@@ -1,7 +1,37 @@
 typescript-service
 ===
+Language service which helps to get diagnostic messages from typescript source files.
 
+## INSTALL
+```
+npm install --save-dev typescript-service
+```
 
-CHANGELOG
----
+## USAGE
+```ts
+import { createService } from 'typescript-service';
+
+const service = createService({ configFile, compilerOptions });
+const diagnostics = service.getDiagnostics(fileName);
+```
+
+## API
+```
+createService({ configFile, compilerOptions })
+```
+`configFile` (required, string) Path to tsconfig.json file  
+`compilerOptions` (optional, Object) Compiler options to overwrite defined in tsconfig.json  
+
+Returns object with properties which are functions:
+
+* `update({ fileName: string, fileContent?: string })` Update (add) information about file in typescript service  
+`fileName` (required, string) Path to typescript file  
+`fileContent (optional, string)` File content of this file  
+
+* `getDiagnostics(fileName: string): Array<ts.DiagnosticWithLocation>` Get diagnostic messages for `fileName`  
+`fileName` (required, string) Path to typescript file  
+
+* `getProgram(): ts.Program` Get `ts.Program`  
+
+## CHANGELOG
 See [CHANGELOG.md](CHANGELOG.md)