Update README

Document new directory structure.
Specify required compiler versions.

Author: blapie

README

@@ -17,14 +17,35 @@ release is v24.05.
 Source code layout:
 
 build/          - build directory (created by make).
-math/           - math subproject sources.
+math/           - math subproject sources for generic scalar
+                  subroutines and sources shared with
+                  subdirectories of math/.
+                  All math routines should meet the quality
+                  requirements stated in math/README.contributors,
+                  routines that fail to do so are located in an
+                  experimental/ directory.
+math/aarch64/   - math subproject AArch64-specific sources
+                  and sources shared with subdirectories.
+math/aarch64/advsimd      - AdvSIMD-specific math sources.
+math/aarch64/experimental - Experimental math sources do not
+                            meet quality requirements stated in
+                            math/README.contributors.
+math/aarch64/sve          - SVE-specific math sources.
 math/include/   - math library public headers.
 math/test/      - math test and benchmark related sources.
 math/tools/     - tools used for designing the algorithms.
 networking/     - networking subproject sources.
 networking/include/ - networking library public headers.
 networking/test/ - networking test and benchmark related sources.
 string/         - string routines subproject sources.
+                  All string routines should meet the quality
+                  requirements stated in string/README.contributors,
+                  routines that fail to do so are located in an
+                  experimental/ directory.
+string/<arch>   - <arch>-specific string routines sources for
+                  <arch>=aarch64, and arm.
+string/aarch64/experimental - Experimental string routines which
+                              may not be fully optimized yet.
 string/include/ - string library public headers.
 string/test/    - string test and benchmark related sources.
 
@@ -49,6 +70,13 @@ Or building and testing the math subproject only:
 make all-math
 make check-math
 
+Note on compiler compability/requirement:
+
+SVE routines are always built by default - this means that on AArch64
+GCC >= 10 or LLVM >= 5 are always required for SVE ACLE compatibility.
+There is no explicit check for compatible compiler, therefore the SVE
+routines will fail to build if CC is too old.
+
 The test system requires libmpfr and libmpc.
 For example on debian linux they can be installed as: