add note to chatSession.sendAttachment in README; addresses #100

[spencerlepine]

Issue #, if available: https://github.com/amazon-connect/amazon-connect-chatjs/issues/100

Description of changes: Add note to chatSession.sendAttachment README documentation

❌ BEFORE: https://github.com/amazon-connect/amazon-connect-chatjs/tree/11edb40b953f8f40b51913d0e5974eca1647aedd#chatsessionsendattachment

✅ AFTER: https://github.com/amazon-connect/amazon-connect-chatjs/tree/spenlep/attachment-documentation-update#chatsessionsendattachment

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[spencerlepine]

Merging, got approvals internally

[senghuotlay]

@spencerlepine how would u do it on react-native as react native doesn't have File type, and i am currently using react-native-image-picker therefore the file type is this

    base64?: string;
    uri?: string;
    width?: number;
    height?: number;
    fileSize?: number;
    type?: string;
    fileName?: string;
    duration?: number;
    bitrate?: number;
    timestamp?: string;
    id?: string;
}```