Update gitlabci/readme with GCC 10 & CLANG 10

* Add clang 10, gcc 10 build in gitlab-ci.
* Make only debug build automatic
* Readme support section updated
4 jobs for !99 with development/clang10 in 13 minutes (queued for 4 seconds)
detached
Status Job ID Name Coverage
  Build
passed #91819
distcheck-linux-clang10-release

00:06:24

passed #91821
distcheck-linux-coverage

00:09:56

90.8%
passed #91820
distcheck-linux-gcc10-release

00:12:59

passed #91818
windows
windows-debug

00:03:24