Added Building contract chapter.

Author: DariuszDepta

docs/tutorial/writing-contracts/building-contract.md

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+---
+sidebar_position: 3
+---
+
+# Building the contract
+
+Now it is time to build our contract. We can use a traditional cargo build pipeline for local
+testing purposes: `cargo build` for compiling it and `cargo test` for running all tests (which we
+don't have yet, but we will work on that soon).
+
+However, we need to create a Wasm binary to upload the contract to blockchain. We can do it by
+passing an additional argument to the build command:
+
+```shell title="terminal"
+cargo build --target wasm32-unknown-unknown --release
+```
+
+The `--target` argument tells cargo to perform cross-compilation for a given target instead of
+building a native binary for an operating system it is running on, in this case, `wasm32-unknown-unknown`,
+which is a fancy name for the Wasm target.
+
+Additionally, we have passed the `--release` argument to the command. It is not required, but in most cases,
+debug information is not very useful while running on-chain. It is crucial to reduce the uploaded binary size
+for gas cost minimization. It is worth knowing that there is a [CosmWasm Rust Optimizer] tool that takes care
+of building even smaller binaries. For production, all the contracts should be compiled using this tool,
+but now, for learning purposes, it is not an essential thing to do.
+
+## Aliasing the build command
+
+Now I can imagine you are disappointed in building your contracts with some overcomplicated command
+instead of simple `cargo build`. Fortunately, it is not the case. The common practice is to alias the building
+command to make it as simple as building a native app.
+
+Let's create the `.cargo/config.toml` file in your contract project directory with the following content:
+
+```toml title=".cargo/config.toml"
+[alias]
+wasm = "build --target wasm32-unknown-unknown --release"
+wasm-debug = "build --target wasm32-unknown-unknown"
+```
+
+Now, building your Wasm binary is as easy as executing `cargo wasm`.
+We also added the additional `wasm-debug` command for very rare cases when you might want
+to build the Wasm binary with debug information.
+
+## Checking contract validity
+
+Once the contract is built, the last step to ensure it is a valid CosmWasm contract is by calling `cosmwasm-check` on it:
+
+```shell title="terminal"
+cargo wasm && cosmwasm-check ./target/wasm32-unknown-unknown/release/contract.wasm
+```
+
+```text title="terminal"
+Available capabilities: {"cosmwasm_1_2", "cosmwasm_1_4", "stargate", "cosmwasm_1_1", "cosmwasm_2_1", "cosmwasm_1_3", "cosmwasm_2_0", "staking", "iterator"}
+
+./target/wasm32-unknown-unknown/release/contract.wasm: pass
+
+All contracts (1) passed checks!
+```
+
+[CosmWasm Rust Optimizer]: https://github.com/CosmWasm/rust-optimizer