doc: add qrencode to brew install instructions #12850

Build was missing qr code(qrencode libs) support and brew team no longer supports anything less than macOS 10.11

Where are you seeing brew no longer supporting macOS < 10.11 ?

On the brew website I see:

10.10 or higher is recommended. 10.5–10.9 are supported on a best-effort basis. 
For 10.4 see Tigerbrew.

I see it when I run it in 10.8 - 10.10 using VM installs of 10.8, 10.9 and 10.10

Also, ssl certs are not maintained anymore by apple and thus the ruby installer barely works if at all. There are workarounds but they are a pain in the behind.

The brew team has not updated their website to reflect what is in their code.

To reproduce the issue you will need to install each version of Mac OS X to confirm via VM or on an older machine. I will leave another comment later with screen shot soon.

@buddilla Thanks for the input, I'll put together some notes about EOL'ing support for macOS releases. However can you remove the changes you've made to the "Tested on" line. Our gitian built binaries still target a minimum of OS X 10.8.

Changed I also added notes that 10.8 is still supported in gitian.

I just realized that I misread what you wanted me to do(I just woke up at the time). Do you want me to revert the changes for the line "Tested on" back to Mac OS X 10.8... Or are the new changes better. The new changes may be better to help avoid further confusion for others, which means less issue posts. But this is your house so whatever you say goes. LOL

I went ahead and changed it back below is what I added previously

• Works with macOS 10.11 through 10.13 on 64-bit Intel processors only.

• macOS 10.8 and higher is still supported on gitian builds

Please squash your commits and update the commit message to something like "doc: add qrencode to brew install instructions"

I am having a heck of a time trying to squash this. "Allow edits from maintainers" is checked. Any help would be appreciated.

Squashed