You have several options, shown in order of increasing effort and degree-of-detail:

• Write a blog for your decisions, every decisions becomes a blog post.
• Use a text format similar to an (Alexandrian) pattern, like explained in question C-9-3 (Architecture Decision Record).
• For important decisions, the following topics might be interesting:
  • subject of the decision
  • affected stakeholders
  • decision criteria (with priorities)
  • alternatives
  • evaluation of alternatives for the various criteria
  • who took the decision?
  • reason for chosing this alternative in favor of others