Document CFLAGS, CXXFLAGS, ASFLAGS and CPPFLAGS.

After understanding the difference between CXXFLAGS and CPPFLAGS, this
is a revised commit for https://github.com/sudar/Arduino-Makefile/pull/257

arduino-mk-vars.md

@@ -852,6 +852,82 @@ CXXFLAGS_STD = = -std=gnu++98
 
 ----
 
+### CFLAGS
+
+**Description:**
+
+Flags passed to compiler for files compiled as C. Add more flags to this
+variable using `+=`.
+
+Defaults to all flags required for a typical build.
+
+**Example:**
+
+```Makefile
+CFLAGS += -my-c-only-flag
+```
+
+**Requirement:** *Optional*
+
+----
+
+### CXXFLAGS
+
+**Description:**
+
+Flags passed to the compiler for files compiled as C++. Add more flags to this
+variable using `+=`.
+
+Defaults to all flags required for a typical build.
+
+**Example:**
+
+```Makefile
+CXXFLAGS += -my-c++-onlyflag
+```
+
+**Requirement:** *Optional*
+
+----
+
+### ASFLAGS
+
+**Description:**
+
+Flags passed to compiler for files compiled as assembly (e.g. `.S` files). Add
+more flags to this variable using `+=`.
+
+Defaults to all flags required for a typical build.
+
+**Example:**
+
+```Makefile
+ASFLAGS += -my-as-only-flag
+```
+
+**Requirement:** *Optional*
+
+----
+
+### CPPFLAGS
+
+**Description:**
+
+Flags passed to the C pre-processor (for C, C++ and assembly source flies). Add
+more flags to this variable using `+=`.
+
+Defaults to all flags required for a typical build.
+
+**Example:**
+
+```Makefile
+CPPFLAGS += -DMY_DEFINE_FOR_ALL_SOURCE_TYPES
+```
+
+**Requirement:** *Optional*
+
+----
+
 ### OVERRIDE_EXECUTABLES
 
 **Description:**