[c/en] Added a section for header files.

Added a section for header files.
Included a discussion of what belongs in a header file and what does
not.

c.html.markdown

@@ -6,7 +6,7 @@ contributors:
     - ["Árpád Goretity", "http://twitter.com/H2CO3_iOS"]
     - ["Jakub Trzebiatowski", "http://cbs.stgn.pl"]
     - ["Marco Scannadinari", "https://marcoms.github.io"]
-
+    - ["Zachary Ferguson", "https://github.io/zfergus2"]
 ---
 
 Ah, C. Still **the** language of modern high-performance computing.
@@ -630,6 +630,54 @@ typedef void (*my_fnp_type)(char *);
 
 ```
 
+Header files are an important part of c as they allow for the connection of c 
+source files and can simplify code and definitions by seperating them into 
+seperate files.
+
+Header files are syntaxtically similar to c source files but reside in ".h" 
+files. They can be included in your c source file by using the precompiler 
+command #include "example.h", given that example.h exists in the same directory 
+as the c file.
+
+```c
+/* A safe guard to prevent the header from being defined too many times. This */
+/* happens in the case of circle dependency, the contents of the header is    */
+/* already defined.                                                           */
+#ifndef EXAMPLE_H /* if EXAMPLE_H is not yet defined. */
+#define EXAMPLE_H /* Define the macro EXAMPLE_H. */
+
+/* Other headers can be included in headers and therefore transitively */
+/* included into files that include this header.                       */
+#include <string.h>
+
+/* Like c source files macros can be defined in headers and used in files */
+/* that include this header file.                                         */
+#define EXAMPLE_NAME "Dennis Ritchie"
+/* Function macros can also be defined. */
+#define ADD(a, b) (a + b)
+
+/* Structs and typedefs can be used for consistency between files. */
+typedef struct node
+{
+    int val;
+    struct node *next;
+} Node;
+
+/* So can enumerations. */
+enum traffic_light_state {GREEN, YELLOW, RED};
+
+/* Function prototypes can also be defined here for use in multiple files,  */
+/* but it is bad practice to define the function in the header. Definitions */
+/* should instead be put in a c file.                                       */
+Node createLinkedList(int *vals, int len);
+
+/* Beyond the above elements, other definitions should be left to a c source */
+/* file. Excessive includeds or definitions should, also not be contained in */
+/* a header file but instead put into separate headers or a c file.          */
+
+#endif /* End of the if precompiler directive. */
+
+```
 ## Further Reading
 
 Best to find yourself a copy of [K&R, aka "The C Programming Language"](https://en.wikipedia.org/wiki/The_C_Programming_Language)