Missing table of contents in the Readme page.

[Nik-09]

Table of contents missing in the top of the readme file, adding a table of contents on the top of the readme file will help in the following:

1. Give a quick summary of what are the contents present on the page.
2. Quickly access any resources without scrolling to the bottom.

Thank you.

[Nik-09]

Before working on it I would like @AdamISZ to take a look into the issue and suggest if any other changes can be added with this.

Thank you.

[AdamISZ]

I suppose it's debatable, but yes adding a table of contents is a pretty good idea, thanks.

Another thing we should add to the README is a link to this new github pages hosting of the documentation:

https://joinmarket-org.github.io/joinmarket-clientserver/

and in particular the documentation of our new RPC-API:

https://joinmarket-org.github.io/joinmarket-clientserver/api/

[Nik-09]

Thank you @AdamISZ for confirming, I will create a pr by end of 7th Feb.