Add how-to-build docs (#172)

docs/guides/how-to-build.md

+# How to build project
+This document helps people to compile and build the project.
+
+## Build Project
+**Because we are using Git submodule, we recommend don't use `GitHub` tag or release page to download source codes for compiling.**
+
+### Build from GitHub
+- Prepare git and .NET Core SDK.
+- `git clone https://github.com/SkyAPM/SkyAPM-dotnet.git`
+- `cd SkyAPM-dotnet/`
+- Switch to the tag by using `git checkout [tagname]` (Optional, switch if want to build a release from source codes)
+- `git submodule init`
+- `git submodule update`
+- Run `dotnet restore`
+- Run `dotnet build src/SkyApm.Transport.Grpc.Protocol`
+- Run `dotnet build skyapm-dotnet.sln`
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