clarify workshop purpose in README

Dependencies

• [2] WQOLDV5Z initial commit: README.md
• [3] YS24ET7A add repository mirrors to README

Change contents

• replacement in README.md at line 3
  [2.51]→[2.51:297](∅→∅)
  − Athough git was created as a decentralised way to collaborate, the practical needs of day-to-day work have pushed most of us to use it through a centralised platform (generically called a "forge"), of which the most common is Microsoft's GitHub.
  [2.51]

  [2.297]
  + Although git was created as a decentralised way to collaborate, the practical needs of day-to-day work have pushed most of us to use it through a centralised platform (generically called a "forge"), of which the most common is Microsoft's GitHub. That centralisation exposes us to a number of risks, primarily reliance on one forge as a single point of failure, so it's useful to have an understanding of what other options are out there.

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  + This workshop will give you a chance to think through and discuss how you use GitHub, and identify options for replacing or augmenting it with other tools to make your development process more resilient.