[gimp-macos-build] Update readme

[Alex Samorukov <asamorukov src gnome org>]

commit 516dd647f26af79bada1eebd3d87af87faa38ddd
Author: Oleksii Samorukov <oleksii_samorukov mckinsey com>
Date:   Sun Oct 27 15:39:08 2019 +0100

    Update readme

 README.md | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)
---
diff --git a/README.md b/README.md
index 5b5c576..4041e99 100644
--- a/README.md
+++ b/README.md
@@ -10,7 +10,13 @@ and some gimp-specific patches to GTK. Currently build is done using CircleCI.
 
 Because CircleCI is not supporting gitlab [yet] there is a [GitHub 
mirror](https://github.com/GNOME/gimp-macos-build) of this repository.
 
-### Steps in the [CircleCI 
config.yml](https://github.com/GNOME/gimp-macos-build/blob/master/.circleci/config.yml) are:
+## Before you starting
+
+I found that GTK and GIMP builds on OSX are very fragile. If you have any other build system (brew, 
macports) installed - try to remove it first or at least isolate from jhbuild env as much as you can.
+
+I was able to get working builds in the VirtualBox VM, it works stable enough for me.
+
+## Steps in the [CircleCI 
config.yml](https://github.com/GNOME/gimp-macos-build/blob/master/.circleci/config.yml) are:
 
 - Install gfortran and rust as they are required for the GIMP dependencies.
 - Setup OSX 10.9 SDK. This is needed to ensure that GIMP is able to run on MacOS 10.9+. See [this 
article](https://smallhacks.wordpress.com/2018/11/11/how-to-support-old-osx-version-with-a-recent-xcode/) for 
the details.
@@ -32,6 +38,7 @@ Because CircleCI is not supporting gitlab [yet] there is a [GitHub mirror](https
 
 ## Branches
 
-- master: latest GIMP release
-- gimp-2-10: gimp-2-10 build
-- debug: same as master, but with full debug symbols
+- `master`: latest GIMP release
+- `gimp-2-10`: gimp-2-10 build
+- `debug`: same as master, but with full debug symbols
+- `hardened-runtime`: singed and notarized package with a hardened runtime enabled