Update profiles format descriptions

Update profiles format descriptions, follow all comments in discuss. Thanks Alex Bradbury and Kito Cheng's review.

Signed-off-by: Jiawei <jiawei@iscas.ac.cn>

README.mkd

@@ -61,25 +61,38 @@ The ISA subset naming conventions are described in Chapter 27 of the RISC-V
 user-level ISA specification. However, tools do not currently follow this 
 specification (input is case sensitive, ...).
 
+If the 'C' (compressed) instruction set extension is targeted, the compiler 
+will generate compressed instructions where possible.
+
 ### Profile-based format
 
-Profiles should be recognized and used in the `-march=` option.
-The benefit is that it is easy for toolchain parsing the profiles string and expanding it into normal extensions combinations. It’s also easy to combine a profile with any extensions.
+Profiles should be recognized and used in the `-march=` option. The benefit use
+`-march` option is easy for toolchain parsing the profiles string and expanding
+it into normal extensions combinations. 
+
+Profiles format has the following form `-march=<profile-name>[+<option-ext>]+`.
 
-As the spec defines, to use the profiles it should follow profile naming convention (See [3.4 form spec doc](https://github.com/riscv/riscv-profiles)), the toolchain will check whether an input profile name is correct at first, then do the parse  work.
+As the spec defines, to use the profiles it should follow profile naming convention
+(See [3.4 form spec doc](https://github.com/riscv/riscv-profiles)), the toolchain
+will check whether an input profile name is correct at first, then do the parse
+work.
 
-In order to distinguish between ordinary extension input and input with profiles, profiles are assumed to be entered `at the beginning of the -march option`, and then input other extensions. Profiles `should use uppercase letters` in the -march option.
+In order to distinguish between ordinary extension input and input with profiles,
+profiles are assumed to be entered `at the beginning of the -march option`, and
+then input other extensions. Profiles `should use uppercase letters` in the `-march`
+option.
 
 e.g. `-march = RVA20U64` is a legal profile input, it will be expand into:
 
-`-march = rv64imafdc_zicsr_ziccif_ziccrse_ziccamoa_zicclsm_za128rs`
+`-march = rv64imafdc_zicsr_ziccif_ziccrse_ziccamoa_zicclsm_za128rs`,
 which include all the mandatory extensions required by this profile.
-Here we are `not dealing with optional extensions` to profiles in the toolchain.
 
-Moreover, some extensions are still not ratified yet. So the toolchain only keeps their name strings only.
+`-march = RVA20U64+zba_zbb_zbc_zbs` is also a legal profile input, it will add
+four new extensions after expanded profile strings:
 
-If the 'C' (compressed) instruction set extension is targeted, the compiler 
-will generate compressed instructions where possible.
+`-march = rv64imafdc_zicsr_ziccif_ziccrse_ziccamoa_zicclsm_za128rs_zba_zbb_zbc_zbs`
+
+and `-march = rva20u64` is an illegal profile input, it not use uppercase letters.
 
 ### Issues for consideration
 * Whether `riscv32` and `riscv64` should be accepted as synonyms for `rv32`