add README to guide followed development

metagpt/environment/README.md

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+Here is a environment description of MetaGPT env for different situation.  
+For now, the code only define the environment and still some todos like migrate roles/actions to current version.
+
+## Function
+- Define `ExtEnv`(Base Class) which help users to integrate with external environment like games through apis or construct the game logics.
+- Define `Environment`(Base Class) which is the env that MetaGPT directly used. And it includes roles and so on.
+- Define the `EnvAPIRegistry` to mark the read/write apis that `ExtEnv` provide observe/step ability. And then, users can call the particular one to get observation from env or feedback to env.
+
+## Usage
+
+init environment
+```
+android_env = env.create(EnvType.ANDROID)
+
+assistant = Role(name="Bob", profile="android assistant")
+team = Team(investment=10.0, env=android_env, roles=[assistant])
+```
+
+observe & step inside role's actions
+```
+from metagpt.environment.api.env_api import EnvAPIAbstract
+
+# get screenshot from ExtEnv
+screenshot_path: Path = env.observe(
+            EnvAPIAbstract(
+                api_name="get_screenshot", kwargs={"ss_name": f"{round_count}_before", "local_save_dir": task_dir}
+            )
+        )
+
+# do a `tap` action on the screen
+res = env.step(EnvAPIAbstract("system_tap", kwargs={"x": x, "y": y}))
+```
+
+## TODO
+- add android app operation assistant under `examples/android_assistant`
+- migrate roles/actions of werewolf game from old version into current version
+- migrate roles/actions of mincraft game from old version into current version
+- migrate roles/actions of stanford_town game from old version into current version