Add document for -mstrict-align and -mno-strict-align

We have those two option for a while, but not document in this repo yet,
however we are intend to claify the scope of the option recently, so
it's time to document down what we have agree among different community.

Author: kito-cheng

README.mkd

@@ -398,11 +398,34 @@ specification, and in the draft [CORE-V Builtin
 Function](https://github.com/openhwgroup/core-v-sw/blob/master/specifications/corev-builtin-spec.md)
 specification.
 
+## Common Toolchain Command Line Options
+
+This section lists common RISC-V specific toolchain command line options.
+
+### `-mstrict-align`/`-mno-strict-align`
+Indicates that the compiler should not assume that unaligned scalar and
+unaligned vector memory references are handled by the system.
+
+`-mstrict-align`: The compiler disallows misaligned memory access.
+`-mno-strict-align`: The compiler allows misaligned memory access.
+
+The compiler's behavior will follow this order of precedence:
+
+- Use the setting from `-mstrict-align` / `-mno-strict-align` if either option
+  is given, taking the last one specified.
+- Use the setting from `-mtune` if `-mstrict-align` / `-mno-strict-align` is not given.
+- Use the setting from `-mcpu` if neither of the above options is given.
+- Use the compiler's default setting if none of the above options are provided.
+
+NOTE: Non-strict also known as unaligned access or misaligned access
+NOTE: The compiler may generate misaligned access if the program violates the
+      alignment assumption.
+NOTE: This option does not affect inline assembly.
+
 ## TODO
 
-* `-mdiv`, `-mno-div`, `-mfdiv`, `-mno-fdiv`, `-msave-restore`, 
-  `-mno-save-restore`, `-mstrict-align`, `-mno-strict-align`, 
-  `-mexplicit-relocs`, `-mno-explicit-relocs`
+* `-mdiv`, `-mno-div`, `-mfdiv`, `-mno-fdiv`, `-msave-restore`,
+  `-mno-save-restore`, `-mexplicit-relocs`, `-mno-explicit-relocs`
 
 ## Appendix: Exposing a vendor-specific extension across the toolchain