README.md: update links to mailinglists

Signed-off-by: Axel Heider <axelheider@gmx.de>

trivial: fix broken links in README.md

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

README.md: Update with links from seL4 docsite

https://docs.sel4.systems/projects/sel4/ contains several links that are
helpful to link to from the seL4 readme.

Signed-off-by: Kent McLeod <Kent.Mcleod@data61.csiro.au>

extract license remark into own file

github detects LICENSE.md files and links to them, so put the license
information there instead of the README.

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

update license information in README

* Point to `LICENSES/` and include the syscall note in the README.
* BSD and GPL license files now provided in LICENSES directory.

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

README: fix build instructions link

SELFOUR-1079: Add link to hosted manual in README

Update DOI link in README.md

Previously the DOI link was to one very old version. Update with a link
that links to the latest version automatically.

Remove outdated build instructions

These instructions do not presently work and should not be used

README.md: Update README for serial output

README.md: Update README for verified platform

Drop Haskell kernel from README

“It's too late to correct it,” said the Red Queen: “when you've once
said a thing, that fixes it, and you must take the consequences.”

update README links

reported by @watbe [https://github.com/seL4/seL4/issues/17]

add DOI

fix typo; add more links

Add links to "seL4.systems" website to "README.md".

prefer markdown for github