[netcore] Add a README. (mono/mono#15934)

* [netcore] Add a README.

Commit migrated from https://github.com/mono/mono/commit/6f0519266903983c28d159a8e9154e02a1467cc4

diff --git a/src/mono/netcore/README.md b/src/mono/netcore/README.md
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+# Introduction
+
+Netcore support in mono consists of two parts:
+* The runtime compiled in netcore mode
+* An implementation of System.Private.CoreLib
+
+# Building
+
+Everything below should be executed with the current dir set to 'netcore'.
+
+For bootstrap, do
+       ./build.sh
+
+To rebuild the runtime, do
+       make runtime
+
+To rebuild System.Private.CoreLib, do
+       make bcl
+
+These two targets will copy the results into shared/Microsoft.NETCore.App/<version>.
+
+# Running with netcore
+
+## Running through the 'dotnet' tool.
+
+Run ```dotnet publish -c Release -r osx-x64``` to create a published version of the app.
+Copy
+```mono/mini/.libs/libmonosgen-2.0.dylib```
+into
+```bin/netcoreapp3.0/osx-x64/publish/libcoreclr.dylib```
+Copy
+```netcore/System.Private.CoreLib/bin/x86/System.Private.CoreLib.{dll,pdb}```
+to
+```bin/netcoreapp3.0/osx-x64/publish```
+
+## Running with the mono runtime executable
+
+DYLD_LIBRARY_PATH=shared/Microsoft.NETCore.App/<dotnet version> MONO_PATH=shared/Microsoft.NETCore.App/<dotnet version> ../mono/mini/mono-sgen --assembly-loader=strict sample/HelloWorld/bin/netcoreapp3.0/HelloWorld.dll