Update README.md

[Darkslategray-lgtm]

This commit adds new options to the go-callvis tool, allowing users to customize the rendering of the call graph. Specifically, users can now select different algorithms to construct the graph, enable caching to avoid unnecessary re-rendering, and adjust the shape and style of the graph nodes. These options enhance the overall flexibility and usability of the tool.

These options are useful, and I recommend putting them in README.md , rather than requiring users to use -h to know about them.