Adds description of gcc problem to documentation (#881)

* Adds description of gcc problem to documentation

* Fixes space issues

* Fix Typo

docs/installation.rst

@@ -100,3 +100,11 @@ Go into the tardis/montecarlo directory and build montecarlo.c by hand::
     cython montecarlo.pyx
 
 Then, ``python setup.py build`` should run without problems.
+
+
+**Problem:** when trying to set up CC=gcc python setup.py develop --with-openmp the following error popped up: 
+from tardis/_compiler.c:1: /Users/yssavo/miniconda2/envs/tardis-show2/lib/gcc/x86_64-apple-darwin13.4.0/5.2.0/include-fixed/limits.h:168:61: fatal error: limits.h: No such file or directory 
+
+**Solution:** Run on terminal: 
+
+    open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg