Add documentation for GRPC_ environment variables

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+gRPC environment variables
+--------------------------
+
+gRPC C core based implementations (those contained in this repository) expose
+some configuration as environment variables that can be set.
+
+* GRPC_ABORT_ON_LEAKS
+  A debugging aid to cause a call to abort() when gRPC objects are leaked past
+  grpc_shutdown()
+
+* GRPC_GOOGLE_CREDENTIALS_ENV_VAR
+  The path to find the credentials to use when Google credentials are created
+
+* GRPC_SSL_CIPHER_SUITES
+  A colon separated list of cipher suites to use with OpenSSL
+
+* GRPC_POLL_STRATEGY [posix-style environments only]
+  Declares which polling engines to try when starting gRPC.
+  This is a comma-separated list of engines, which are tried in priority order
+  first -> last.
+  Available polling engines include:
+  - epoll (linux-only) - a polling engine based around the epoll family of
+    system calls
+  - poll - a portable polling engine based around poll(), intended to be a
+    fallback engine when nothing better exists
+  - legacy - the (deprecated) original polling engine for gRPC
+
+* GRPC_TRACE
+  A comma separated list of tracers that provide additional insight into how
+  gRPC C core is processing requests via debug logs. Available tracers include:
+  - api - traces api calls to the C core
+  - channel - traces operations on the C core channel stack
+  - combiner - traces combiner lock state
+  - compression - traces compression operations
+  - connectivity_state - traces connectivity state changes to channels
+  - channel_stack_builder - traces information about channel stacks being built
+  - http - traces state in the http2 transport engine
+  - http1 - traces HTTP/1.x operations performed by gRPC
+  - flowctl - traces http2 flow control
+  - op_failure - traces error information when failure is pushed onto a
+    completion queue
+  - pending_tags - [debug builds only] traces still-in-progress tags on
+    completion queues
+  - round_robin - traces the round_robin load balancing policy
+  - glb - traces the grpclb load balancer
+  - queue_pluck
+  - queue_timeout
+  - secure_endpoint - traces bytes flowing through encrypted channels
+  - transport_security - traces metadata about secure channel establishment
+  - tcp - traces bytes in and out of a channel
+  'all' can additionally be used to turn all traces on.
+  Individual traces can be disabled by prefixing them with '-'.
+  Example:
+   export GRPC_TRACE=all,-pending_tags
+
+* GRPC_VERBOSITY
+  Default gRPC logging verbosity - one of:
+  - DEBUG - log all gRPC messages
+  - INFO - log INFO and ERROR message
+  - ERROR - log only errors
+