Update packages.rst

Explains to contributor that list of available packages will be automatically updated.
Merge pull request #2851 from jiahao/patch-1

Update packages.rst
Slight updates on the doc regarding package list auto-generation
c_CholmodFactor and CholmodFactor depend on version of CHOLMOD

In version 2.1.0 of the CHOLMOD library of C functions a new field was
added to the cholmod_factor struct.  For compatibility the definitions
of the c_CholmodFactor and CholmodFactor types now depend on the
version of the CHOLMOD headers through the value of a trivial C
function jl_cholmod_version defined in deps/Suitesparse_wrapper.c

This is not ideal.  It would be better for the shared object file
libcholmod.so to contain such a function so that the version of the
code in the shared object can be checked and not just the version of
the headers against which deps/Suitesparse_wrapper.c was compiled.

The suitesparse tests are reinstated.  Also mistakes related to the x
and px fields in the external constructor for the CholmodFactor object
from Ptr{c_CholmodFactor} were corrected and the code for the logdet
methods was changed to use the diag extractor.  This allows the logdet
of supernodal factorizations to be evaluated.
Remove duplicate of external constructor for CholmodFactor
Update packages.rst

Encourages contributors to contribute to existing packages whenever sensible,  and lists instructions to check for existing packages.
Merge pull request #2854 from jiahao/patch-1

Updated package contribution guidelines
Pkg: be crystal clear about the branch location

Prevents the following error:

julia> Pkg.update()
warning: refname 'origin/devel' is ambiguous.
fatal: Ambiguous object name: 'origin/devel'.
ERROR: failed process: Process(`git branch -f devel origin/devel`, ProcessExited(128)) [128]
 in error at error.jl:22
 in pipeline_error at process.jl:394
 in run at process.jl:384
 in anonymous at no file:567
 in cd at file.jl:25
 in anonymous at no file:564
 in cd at file.jl:25
 in cd_pkgdir at pkg.jl:42
 in update at pkg.jl:563