[docs] add libatomic as a required dependency (#25061)

Author: vchuravy

README.md

@@ -280,15 +280,16 @@ Julia can be developed in an isolated Vagrant environment. See [the Vagrant READ
 Building Julia requires that the following software be installed:
 
 - **[GNU make]**                — building dependencies.
-- **[gcc & g++][gcc]** (>= 4.7) or **[Clang][clang]** (>= 3.1, Xcode 4.3.3 on OS X) — compiling and linking C, C++
-- **[python]** (>=2.7)          - needed to build LLVM.
+- **[gcc & g++][gcc]** (>= 4.7) or **[Clang][clang]** (>= 3.1, Xcode 4.3.3 on OS X) — compiling and linking C, C++.
+- **[libatomic][gcc]**          — provided by **[gcc]** and needed to support atomic operations.
+- **[python]** (>=2.7)          — needed to build LLVM.
 - **[gfortran]**                — compiling and linking Fortran libraries.
 - **[perl]**                    — preprocessing of header files of libraries.
 - **[wget]**, **[curl]**, or **[fetch]** (FreeBSD) — to automatically download external libraries.
 - **[m4]**                      — needed to build GMP.
 - **[patch]**                   — for modifying source code.
 - **[cmake]** (>= 3.4.3)        — needed to build `libgit2`.
-- **[pkg-config]**              - needed to build `libgit2` correctly, especially for proxy support.
+- **[pkg-config]**              — needed to build `libgit2` correctly, especially for proxy support.
 
 Julia uses the following external libraries, which are automatically downloaded (or in a few cases, included in the Julia source repository) and then compiled from source the first time you run `make`: