Fix math rendering in readme

[shaido987]

Hello,

There are some issues with the math not rendering correctly when looking at the readme file. Currently, it is shown as code with surrounding $ signs which is very hard to read, for example, $w$. This PR should fix the issue.

Some additional notes:

• The \text{ignoreindex} have issues rendering in github markdown due to the '' so I had to replace it with \text{ignore}\_\text{index}, see: https://groups.google.com/g/mathjax-users/c/wSh6-hSIUpQ/m/d53b0sB1SKcJ

• Directly using a sum in the denominator of a fraction renders incorrectly so I had to replace \sum with \Sigma, visually it looks the same.

• The line $\text{ignore\_index} \in C \cup \{\bot\} \in$ ends with a \in so the math does not render at all. I removed the \in to read $\text{ignore}\_\text{index} \in C \cup {\bot}$ but if the dimensions should be shown it needs to be readded.

[chenkins]

@shaido987 Thanks for pointing out. Continuing in #5

[shaido987]

Happy to help. Ah, there looks to be some strange renderings that I missed. I will see if I can figure them out as well.