Expand description of get_plugins() in README

[sweep-ai[bot]]

This PR expands the description of the get_plugins() function in the README. The updates include:

• Adding more details on the filter, category, and provider arguments
• The filter argument can be used to filter plugins by verification status. Options are:
  • 'working' - Returns only plugins that have been tested and are working.
  • 'ChatGPT' - Returns only plugins verified to work with ChatGPT.
• The category argument can be used to filter plugins by category. Options are:
  • 'travel'
  • 'ecommerce'
  • 'knowledge'
  • 'etc.'
• You can get a full list of category names using the get_category_names() function.
• The provider argument specifies which provider to get plugins from. Options are 'plugnplai' or 'pluginso'.

These updates provide readers with more clarity on how to use the get_plugins() function.

Fixes #133.