Update README.md to make clear to use different strategies to optimize the process

Note that as far as I (and StackOverflow ([1.](https://stackoverflow.com/q/63387215) and [2.](https://stackoverflow.com/q/67652250)) seems to) know there is no workaround to the 20,000 limit of PlaylistItems: list. This issue can be checked with:

```py
import requests, json

PLAYLIST_ID = 'UUf8w5m0YsRa8MHQ5bwSGmbw'
API_KEY = 'AIzaSy...'

items = []
pageToken = ''
while True:
    url = f'https://www.googleapis.com/youtube/v3/playlistItems?part=id&playlistId={PLAYLIST_ID}&maxResults=50&key={API_KEY}&pageToken={pageToken}'
    content = requests.get(url).text
    data = json.loads(content)
    items += data['items']
    print(len(items))
    if 'nextPageToken' in data:
        pageToken = data['nextPageToken']
    else:
        break

print(len(items))
```

Returns >= 19,000.

Note that this algorithm says that:
- [france24](https://www.youtube.com/@FRANCE24) has 6,086 videos while [SocialBlade states that it has 101,196 videos](https://socialblade.com/youtube/user/france24)
- [CNN](https://www.youtube.com/@CNN) has 19,289 while [SocialBlade states that it has 157,321 videos](https://socialblade.com/youtube/user/cnn)

Indeed both YouTube Data API v3 Search: list (I verified that https://github.com/Benjamin-Loison/YouTube-operational-API/issues/4 applied here with below code) and web-scraping `VIDEOS` tab don't work (see second SO link).

```py
import requests, json

CHANNEL_ID = 'UCf8w5m0YsRa8MHQ5bwSGmbw'
API_KEY = 'AIzaSy...'

items = []
pageToken = ''
while True:
    url = f'https://www.googleapis.com/youtube/v3/search?part=id&type=video&channelId={CHANNEL_ID}&maxResults=50&key={API_KEY}&pageToken={pageToken}'
    content = requests.get(url).text
    data = json.loads(content)
    items += data['items']
    print(len(items))
    if 'nextPageToken' in data:
        pageToken = data['nextPageToken']
    else:
        break

print(len(items))
```

Got ~18,734.

Another try by working with Search: list with date filter may make sense.

Note that according to SocialBlade:
- [asianetnews has 195,600 videos](https://socialblade.com/youtube/user/asianetnews)
- [RoelVandePaar has 2,2025,566 videos](https://socialblade.com/youtube/c/roelvandepaar)

README.md

@@ -1,9 +1,12 @@
 As explained in the project proposal, the idea to retrieve all video ids is to start from a starting set of channels, then list their videos using YouTube Data API v3 PlaylistItems: list, then list the comments on their videos and then restart the process as we potentially retrieved new channels thanks to comment authors on videos from already known channels.
 
 For a given channel, there is a single way to list comments users published on it:
-As explained, YouTube Data API v3 PlaylistItems: list endpoint enables us to list the channel videos **(isn't there any limit?)** and CommentThreads: list and Comments: list endpoints enable us to retrieve their comments
+As explained, YouTube Data API v3 PlaylistItems: list endpoint enables us to list the channel videos up to 20,000 videos and CommentThreads: list and Comments: list endpoints enable us to retrieve their comments
 
 We can multi-thread this process by channel or we can multi-thread per videos of a given channel.
 As would like to proceed channel per channel, the question is **how much time does it take to retrieve all comments from the biggest YouTube channel? If the answer is a long period of time, then multi-threading per videos of a given channel may make sense.**
 
+**In fact should proceed fastly with CommentThreads: list with `allThreads...` when possible**
+**do I have an example of channels where commentthreads: list work but doesn't list a comment of a video ... ?**
+
 Have to proceed with a breadth-first search approach as treating all *child* channels might take a time equivalent to treating the whole original tree.