adding release notes, 2.1

ReleaseNotes2_1.md

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+#Release Notes for EPANET 2.1
+
+The last update to the EPANET engine was "Build 2.00.12" in February of 2008. Since that time, a community effort to update and extend the open-source code has emerged. This group has made a number of bug-fixes and API additions that help to improve the EPANET engine for everyone. Version 2.1 is now released after 8.5 years.
+
+Contributors to this version (listed in order of first contribution):
+
+- Lew Rossman
+- Michael Tryby
+- Feng Shang
+- James Uber
+- Tom Taxon
+- Sam Hatchett
+- Hyoungmin Woo
+- Jinduan Chen
+- Yunier Soad
+- Mike Kane
+- Demetrios Eliades
+- Will Furnass
+- Steffen Macke
+- Mariosmsk
+- Elad Salomons
+- Maurizio Cingi
+- Bryant McDonnell
+
+##API Additions (new functions):
+- `ENgetaveragepatternvalue`
+- `ENgetstatistic`
+- `ENgetcoord / ENsetcoord`
+- `ENgetpumptype`
+- `ENgetqualinfo`
+
+###Demands
+- `ENgetnumdemands`
+- `ENgetbasedemand / ENsetbasedemand`
+- `ENgetdemandpattern`
+
+###Curves
+- `ENgetcurve`
+- `ENgetcurveid`
+- `ENgetcurvelen`
+- `ENgetcurvevalue`
+- `ENsetcurvevalue`
+- `ENsetcurve`
+- `ENaddcurve`
+- `ENgetheadcurveindex`
+- `ENgetcurveindex`
+
+
+##API Extensions (additional parameters)
+###node value types:
+- `EN_TANKVOLUME`
+- `EN_MAXVOLUME`
+
+###link value types:
+- `EN_LINKQUAL`
+- `EN_LINKPATTERN`
+
+###time parameters:
+- `EN_STARTTIME`
+- `EN_HTIME`
+- `EN_QTIME`
+- `EN_HALTFLAG`
+- `EN_NEXTEVENT`
+
+###(new) statistic values:
+- `EN_ITERATIONS`
+- `EN_RELATIVEERROR`
+
+###pump types
+- `EN_CONST_HP`
+- `EN_POWER_FUNC`
+- `EN_CUSTOM`
+
+##Notable Performance Improvements, Bug Fixes, Usage Features, and other notes
+- updated hash table algorithm
+- fixed memory leak when saving output
+- enables interleaved hydraulic and water quality analysis steps:
+
+```
+	ENopenH();
+	ENinitH(0);
+	ENinitQ(EN_NOSAVE);
+	do {
+	  ENrunH(&t);
+	  ENrunQ(&qt);
+	  ENnextQ(&qstep);
+	  // collect results
+	  ENnextH(&tstep);
+	} while (tstep > 0);
+	ENcloseQ();
+	ENcloseH();
+```
+
+- engine code and command-line executable are now in separate implementation files
+- parameter `#define` directives are now enumerated values
+- main header now contains doxygen-compatible comment blocks for auto-generated documentation
+
+
+
+
+