Clarify maintenance status on README

[alamb]

I have seen several PRs with major API / feature improvements such as #407 #393 #351 #345

Realistically I don't have the bandwidth to manage such changes, and this crate is working fine for most current users. Thus I want to make it clear in the README I don't plan to accept / review any major future changes

cc @Dandandan @houqp @andygrove @nickolay

[coveralls]

Pull Request Test Coverage Report for Build 1813190442

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 165 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.007%) to 90.533%

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Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_common.rs	19	98.98%
src/ast/ddl.rs	23	81.53%
src/parser.rs	123	84.29%

Totals
Change from base Build 1813028528:	0.007%
Covered Lines:	7000
Relevant Lines:	7732

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💛 - Coveralls