BED-4720 - Update installation/upgrade docs

[StephenHinck]

Description

Updates README.md to more clearly communicate how to install and upgrade BloodHound CE. Previously, the one-liner could result in users not getting the latest version of BloodHound if the command was run multiple times in a row due to how Docker image caching works.

Motivation and Context

This PR addresses: [GitHub issue or Jira ticket number]

Insure users know how to install and upgrade BloodHound consistently to have the latest version.

How Has This Been Tested?

Validated instructions locally.

Screenshots (optional):

Types of changes

• Chore (a change that does not modify the application functionality)

Checklist:

• I have met the contributing prerequisites
  • Assigned myself to this PR
  • Added the appropriate labels
  • Associated an issue: Issues and Pull Requests README #672
  • Read the Contributing guide: https://github.com/SpecterOps/BloodHound/wiki/Contributing
• I have ensured that related documentation is up-to-date
  • Open API docs
  • Code comments (GoDocs / JSDocs)
• I have followed proper test practices
  • Added/updated tests to cover my changes
  • All new and existing tests passed

[superlinkx]

Looks good, just an extra . that we probably want to move outside of the code tags for easier copy/paste. Will approve once that's fixed