Added file write-up

tools/file/README.md

@@ -0,0 +1,18 @@
+# File command
+
+`file` returns the detected type of a file.
+
+# Use
+
+Pass any file as a parameter for `file` to get the filetype
+
+```
+$ file file.c
+file.c: C program text
+```
+
+# More
+
+* `man file`
+
+* [file](https://en.wikipedia.org/wiki/File_command)

topics/README.md

@@ -8,7 +8,7 @@ CTF challenges are usually categorized into one of these broad groups, and altho
 
 3. If the file is a jumbled text file, it is most likely a [cryptography](./cryptography/) challenge.
 
-4. If the file provided is not readily identifiable, the best tool to use is the `file` command in the command line, which tells you what type of file it is.
+4. If the file provided is not readily identifiable, the best tool to use is the [file](../tools/file/README.md) command, which tells you what type of file it is.
 
  * If the file output is PCAP or relating to packets, the challenge is a [web](./web) one.