README: Add badge for support chat

[melvo]

PR check list:

• [x] Document your code
• [x] Add \since QXmpp 1.X
• [x] Fix doxygen warnings (see log when building with -DBUILD_DOCUMENTATION=ON)
• [x] Update doc/xep.doc
• [x] Add unit tests
• [x] Format the code: Run clang-format -i src/<edited-file(s)> tests/<edited-file(s)>

[codecov[bot]]

Codecov Report

Merging #437 (81bb567) into master (35ea542) will decrease coverage by 6.73%. The diff coverage is n/a.

:exclamation: Current head 81bb567 differs from pull request most recent head bb091b4. Consider uploading reports for the commit bb091b4 to get more accurate results

@@            Coverage Diff             @@
##           master     #437      +/-   ##
==========================================
- Coverage   76.02%   69.29%   -6.74%     
==========================================
  Files         241      254      +13     
  Lines       20729    23377    +2648     
==========================================
+ Hits        15760    16198     +438     
- Misses       4969     7179    +2210     

Impacted Files	Coverage Δ
src/base/QXmppUtils.cpp	86.66% <0.00%> (-13.34%)	:arrow_down:
src/base/QXmppFutureUtils_p.h	79.24% <0.00%> (-10.56%)	:arrow_down:
src/client/QXmppPubSubManager.h	96.55% <0.00%> (-3.45%)	:arrow_down:
tests/util.h	97.50% <0.00%> (-2.50%)	:arrow_down:
tests/qxmppomemodata/tst_qxmppomemodata.cpp	98.24% <0.00%> (-0.01%)	:arrow_down:
src/base/QXmppMessage.h	100.00% <0.00%> (ø)
src/base/QXmppPubSubEvent.h	100.00% <0.00%> (ø)
src/client/QXmppTrustMemoryStorage.cpp	100.00% <0.00%> (ø)
src/client/QXmppAtmTrustMemoryStorage.cpp	100.00% <0.00%> (ø)
src/base/QXmppOmemoData.cpp
... and 19 more

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[lnjX]

this doesn't look nice (the line break inside of the heading)

[melvo]

@lnjX It is fixed now.