README.md: document how to install from source

[leoluk]

Stack:

• Use emojis for "spr status" output #155
• Support the "gh" CLI tool in addition to "hub" #156
• Only update run count when user is not already a stargazer #154
• Do not panic on runtime errors #153
• Manual merge notice #152
• README.md: document how to install from source #151 ⮜
• Wrap commit body in code area in PR message #150
• Mark current position in stack #149
• Include https:// in PR link #148
• Reformat PR comment messages #147

⚠️ Part of a stack created by spr. Do not merge manually using the UI - doing so may have unexpected results.

[leoluk]

✓ Commit merged in pull request #156