docs: Added a description of Offer Up and instructions to install in ReadMe.md

[ghost]

The ReadMe.md file at the root is edited. Here is a summary:

1. A proper description of the repo was missing from the readme.md so I used a paraphrased version of the description provided in the getting-started tutorial.
2. The link to image "https://docs.agoric.com/assets/img/new_002_small2.2dfb7462.png" was broken. It is replaced with an image of the dapp UI.
3. I expanded a bit on the getting started subsection. A concise version of the instructions to install and start contract are provided. This is again a summarized version of the instructions provided in the tutorial.
4. In the testing section of the readme.md file, instructions to run unit tests and end-to-end tests is provided.
5. The links provided in Contributing subsections were a bit confusing to new devs therefore they are removed.

[dckc]

very nice!