Docs updated
Add "Why SymPy?" section
Remove stuff from the bottom of the Sphinx index

Stuff that can't be reached by the toc is reported by Sphinx, and will be
caught by Travis.  #doctest: +SKIP are automatically stripped by Sphinx
(probably numpydoc), so that is no longer an issue.
Merge pull request #2165 from raoulb/elliptic_meijerg

Add elliptic integrals E and K to the Meijer-G tables
Add note about shifted series
Merge pull request #2202 from skirpichev/3175-karr-sum

Implement Karr definition for summations with reversed limits (issue 3175)
Test for the Karr summation convention
Fixed X and CZ gates
Merge pull request #2203 from raoulb/test_karr_convention

Test for the Karr summation convention
Merge branch 'master' into 3128_Sum_Product_Simplification

Conflicts:
	sympy/concrete/tests/test_sums_products.py
Test failures fixed
polys: A little cleanup of rootisolation.py
Update matrices
(Retroactively) add Acebulf to AUTHORS/aboutus. Welcome to SymPy!
(Retroactively) add Case Van Horsen to AUTHORS/aboutus. Welcome to SymPy!
(Retroactively) add Max Hutchinson to the AUTHORS/aboutus. Welcome to SymPy!
Add hacman to the AUTHORS/aboutus. Welcome to SymPy!
(Retroactively) add Shravas K Rao to the AUTHORS/aboutus file. Welcome to SymPy!
Update .mailmap

Now it and the AUTHORS file are completely up-to-date.
Merge branch 'master' into log_integral