Add the very beginnings of a styleguide (#672)

* Add the very beginnings of a styleguide

* Update STYLEGUIDE.md

* Add some considerations about trailing commas

Co-authored-by: Erick <erickzanardoo@gmail.com>

STYLEGUIDE.md

@@ -0,0 +1,74 @@
+# Style Guide
+
+This is WIP! It is definitely not complete. We just starting putting this together.
+
+## Trailing Commas and Wrapping
+
+List of elements must always be all in one line or one element per line. This includes parameters, arguments, collection literals, etc. Furthermore, if multiline, the last element must have a trailing comma.
+
+For the sake of example, let's use a function invocation (the same apply for all cases):
+
+```
+// good
+foo(p1, p2, p3)
+
+// good
+foo(
+    p1,
+    p2,
+    p3,
+)
+
+// bad: missing trailing comma
+foo(
+    p1,
+    p2,
+    p3
+)
+
+// bad: mixed argument lines
+foo(
+    p1, p2,
+    p3,
+)
+
+// bad: mixed argument lines
+foo(f1,
+    f2)
+```
+
+## Imports
+
+* Never include unused or duplicated imports.
+* You must always use relative imports for imports within the Flame library.
+* Omit `./` for relative imports from the same directory.
+* TODO Do/Do not use the imports grouping internally (like component.dart, extensions.dart, etc).
+* Order your imports by:
+* * Three main blocks, each separated by exactly one empty line:
+* * * Dart SDK dependencies,
+* * * External libraries/Flutter imports,
+* * * Internal (same library) imports.
+* * Then, for each block, order alphabetically.
+* * * For relative imports, that means further away (more `../`) imports will be first.
+
+For example:
+
+```
+import 'dart:math';
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/rendering.dart';
+import 'package:flutter/widgets.dart' hide WidgetBuilder, Viewport;
+import 'package:ordered_set/comparing.dart';
+import 'package:ordered_set/ordered_set.dart';
+
+import '../../extensions.dart';
+import '../components/component.dart';
+import '../components/mixins/has_game_ref.dart';
+import '../components/mixins/tapable.dart';
+import '../components/position_component.dart';
+import '../fps_counter.dart';
+import 'camera.dart';
+import 'game.dart';
+import 'viewport.dart';
+```