saml21: add README stating origin for SAML21 CMSIS includes

cpu/sam0_common/include/cmsis/saml21/README.md

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+# CMSIS from Atmel Software Foundation (ASF)
+
+The include files in the directory tree are copied from ASF.  See
+https://spaces.atmel.com/gf/project/asf/frs/?action=FrsReleaseBrowse&frs_package_id=4
+(dd.  2015-03-12 ASF version 3.22.0 was used)
+
+The directory tree was copied "as is" and its structure is as follows:
+
+    cmsis
+    └── saml21
+        ├── include
+        │   ├── component
+        │   ├── instance
+        │   └── pio
+        └── source
+            ├── gcc
+            └── iar
+
+There is only one include file (per CPU variant) that should be included in
+the source code.  For SAML21 that is cmsis/saml21/include/saml21.h.  But
+that will only work if the proper define is set.  The define is named after
+the variant, for example `__SAML21J18A__`.  This define must be set in the
+`Makefile.include` of the board.
+
+Be aware that if you want to make changes to any file in this tree that the
+changes will be lost when a new ASF release is going to be used.
+
+## Trailing White Space
+
+Because of the whitespace check (dist/tools/whitespacecheck/check.sh) all
+the trailing white space had to be removed.  Please take this into account
+when comparing to the original ASF distribution.
+
+    find include/ -name '*.h' -exec sed -i 's/\s*$//' '{}' +
+
+## LITTLE_ENDIAN
+
+These include files define `LITTLE_ENDIAN`.  But we think this is wrong.  It
+seems more logical to let the compiler decide in which mode the ARM code is
+to be translated.  In include/machine/endian.h there is already a define of
+`LITTLE_ENDIAN` (and `BIG_ENDIAN`) for a different purpose.
+
+So, we decided to remove the define from the ASF CMSIS files.  The command
+for it (running from this directory) is:
+
+    find include/ -name '*.h' -exec sed -i '/^#define\s\s*LITTLE_ENDIAN/d' '{}' +
+