Improve package usage doc

package_usage.md

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+# Documentation
+
+`anime_downloader` is designed to be used in your own package too.
+
+```python
+from anime_downloader import NineAnime
+
+anime = NineAnime('https://www4.9anime.is/watch/dragon-ball-super-dub.r65p', quality='480p')
+
+print(anime.title)
+print(len(anime))
+```
+`quality` can any of `['360p', '480p', '720p']`. If not this line will raise `AnimeDlError`.
+
+```python
+ep = anime[0]
+```
+The type of `ep` is `NineAnimeEpisode`.
+```python
+print(ep.title)
+ep.downloader(path='/home/vn-ki/Downloads/anime/dbs')
+```
+
+### Example to download all episodes of dbs.
+
+```python
+from anime_downloader import NineAnime
+
+anime = NineAnime('https://www4.9anime.is/watch/dragon-ball-super-dub.r65p', quality='480p')
+
+for ep in anime:
+    ep.download()
+```