Add visual header to readme

[wren]

It's just a screenshot from the website, but it's a start to make the readme look a bit nicer. Also, the picture of the terminal quickly shows what jrnl is about.

Note: The img element requires an absolute path here, so I've hardcoded it to the develop branch, so it wont be visible in the preview until this is merged. You can preview the readme file locally, if you check out the branch, and change the url to the local file included in this branch.

Checklist

• I have read the contributing doc.
• I have included a link to the relevant issue number.
• I have tested this code locally.
• I have checked to ensure there aren't other open pull requests
  for the same issue.
• I have written new tests for these changes, as needed.
• All tests pass.

[micahellison]

📔

[github-actions]

Unit Test Results

  16 files  ±0    16 suites  ±0   9s ⏱️ -2s
274 tests  - 6  274 ✔️  - 6  0 💤 ±0  0 ❌ ±0 
275 runs   - 6  275 ✔️  - 6  0 💤 ±0  0 ❌ ±0 

results for commit 514ee30 ± comparison against base commit 8ec661b