README.md: Document generation of SBcAP code from ASN.1 files

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 The current patch queue for osmo-cbc can be seen at
 https://gerrit.osmocom.org/#/q/project:osmo-cbc+status:open
+
+
+Generating asn1c code
+---------------------
+
+Upstream master as1nc from [vlm](https://github.com/vlm/asn1c) [doesn't support
+APER encoding](https://github.com/vlm/asn1c/issues/452). Nevertheless, the
+upstream fork maintained by a big contributor
+[mouse07410](https://github.com/mouse07410/asn1c) does support it, and it is
+used in osmo-cbc to generate the SBc-AP code from ASN.1 files present in
+src/sbcap/asn1/.
+
+In order to regenerate the code, one shall adjust the ASN1C_SKELETON_PATH and
+ASN1C_BIN_PATH in configure.ac to point to the built & installed asn1c from
+mouse07410 (usually `vlm_master` branch). Last generated code was built using
+commit hash 08b293e8aa342d465d26805d1d66f3595b2ce261.
+
+Then, do the usual `autoreconf -fi && ./configure`, using a buildir != srcdir
+(important, in order to avoid ending up with tempotary files in srcdir and
+making it difficult to stash the relevant changes).
+
+Finally, run `make -C src/ regen`, which will regenerate the files and copy over
+the skeletons, with git possibily showing changes in the following paths:
+- include/osmocom/sbcap/
+- src/sbcap/gen/
+- src/sbcap/skel/
+

